This question can be answered from various perspectives. Some of these views imply that this act may have been a product of God: creating a voice, seeing one of God’s luminous creations, and Him manifesting Himself by means of his own creation. Regarding the act of manifestation, although God cannot be perceived by the senses prior to the act of manifestation, after manifestation this is definitely a possibility. In manifestation God is not presenting His true self, therefore, a material display – amongst other things – is definitely a possibility. For further explanation, please refer to the detailed answer below.
The question posed is seemingly simple but very important and a number of responses to it can be provided.
To begin with, ‘Ilm al-Kalaam (the Islamic philosophical discipline of seeking theological principles through dialectic) clearly dictates that God cannot be seen, heard, or perceived through any of our senses whatsoever. Therefore, we must find other means of explaining how it’s possible for God to have spoken with Prophet Moses (a) when he declared, as mentioned in the Holy Qur’aan, “[O’ Moses] Indeed I am your Lord!” Since God cannot be sensed, one could contend that which Prophet Moses (a) heard was not God Himself, but rather a creation which we would recognize as speech and sound. This creation would be similar to the other products of God’s making and capable of conveying His message to Prophet Moses (a). Assuming this was the case, Prophet Moses (a) would not have heard the speech of God Himself (since He is pure of such discordant traits) but rather conversed with and witnessed a product of His creation. A parallel example of this would be hearing a pre-recorded audio bit which we’re certain contains a message from the immaterial realm.
However, this isn’t the strongest argument which could be provided. Though it would be incorrect to consider this argument weak or incorrect, considering the verse’s tone and the apparent understanding of Prophet Moses (a) when he spoke to God, there may appear to be a disparity between what actually happened and the explanation provided. Therefore, a proof must be given which addresses the complete reality behind this incident and its numerous intricacies.
Another response may suggest that Prophet Moses (a) in reality did not hear the voice of God, but rather the sound of one of God’s divine luminous chosen ones. Prophet Moses (a) may have witnessed the light of this creation manifest itself upon Mount Sinai and lost himself not in God, but rather the representative which He created. In any case, it’s important to understand that this light was not God but rather His representative. We must make this distinction of it being other than God in order to, theologically, be able to attribute traits such as sound, light, etc. to ot. However, if there is no direct connection between this creation and the Almighty, then this response would be deemed incorrect. After all, the voice which addressed Prophet Moses (a) laid claim to being God and not a creation of His. In addition, Prophet Moses (a) was certain that he was speaking with God Himself and the Holy Qur’aan attests that God indeed conversed with Prophet Moses (a) directly.
Some Shi‘ah narrations mention that this light which Prophet Moses (a) saw manifest before him was not the light of God, but rather the light of one of his “Karrubin”. Using a different explanation, this claim yet again suggests that the word spoken to Prophet Moses (a) was other than the direct word of God. Although this argument may be true, if used however in response to this fallacy, we would be essentially differentiating between God and His manifestation. As mentioned previously, this isn’t possible because the Holy Qur’aan attests that God conversed with Prophet Moses (a) directly. Of course, if we were to deem this creation as being united in essence with God – as mentioned in Islamic mysticism – this would be a different discussion.
Although we’ve mentioned a number of responses, the central question here still remains unanswered. How could one observe God and His light, or in the case of Prophet Moses (a), ask to witness Him and His beauty upon hearing God’s voice? On one hand, we’re expected to believe that none of these traits can be attributed to God and on the other we see verses of the Holy Qur’aan displaying otherwise. In other words, how can we prove that Prophet Moses’ (a) vision was true and that he really did converse with God while at the same time dispel these negative attributes from God?
Although it is definitely possible that the voice which Prophet Moses (a) heard was in fact a creation of God, we need to discover what distinctiveness it possessed which caused God to attribute the voice to Himself. The association between God and what Prophet Moses (a) observed must be connected in a way to God Himself – not completely detached. Otherwise, we would have no choice but to take the following verse as a metaphorical one: “…and to Moses Allah spoke directly.” However, this cannot be the case. In the words of the Holy Qur’aan, “Immaculate is He, and greatly exalted above what they say!”
This introduction was a necessary perquisite to correctly understanding what Islamic mysticism has to say regarding this matter. At this point, we will attempt to shed light on this verse and its implications by means of this science.
Firstly, note that the main point of debate here is whether or not one is capable of hearing the voice of God. The answer will differ if considered from the perspective of a mystic as opposed to a non-mystic. In other words, the world view of an individual who has become selfless through self-building and Islamic discipline will differ from another person who is still caught up in their self and the distractions of the material world. An example of this can be seen in the story of Prophet Moses (a) when he was commanded to liberate himself of his attachments in order to reach the Divine. If a person considers only himself and is involved, not in the worship of God, but rather that of the material world, that individual will never be able to see God. Conversely, if a person is neither self-involved nor distracted by anything besides God, they will witness - to the extent of their liberty – the manifestations of God in this world and in themselves.
Therefore, the rule perpetually stands – where there are others, there is no [remembrance of] God and where there is God, there is no [remembrance of] others. Even one’s own senses abide by this rule. If a person shifts their awareness from anything other than God and focuses their attention upon the Divine, even their senses can become the station of God’s manifestation. They too can become like Prophet Moses (a), who upon releasing his sandals, heard a voice emanate from the tree before him and declare “Indeed I am your Lord!” When Prophet Moses (a) heard the voice of the Almighty, he had no doubt in terms of what he had seen and heard.
One should know that this act of foregoing the self, worshipping nothing but God, and seeing nothing but Him is not a matter which is purely theoretical. In fact, it is a status divinely bestowed upon those who practically implement the necessary perquisites throughout their everyday lives. Imam ‘Ali (a), who reached the highest of these levels, said that he saw God in everything which he witnessed.
Therefore, concepts such as monotheism, polytheism, etc. abide by this same rule of mysticism – whether it be in the unseen realm or the apparent. Meaning, if a person sacrifices everything but God, they will see His manifestation before them. On the other hand, as much as a person sacrifices their relationship with God [in pursuit of other ambitions], God’s presence in their eyes will become more concealed. And from this mystical concept originates the verse “He is the First and the Last, the Manifest and the Hidden.”
The Prophet Moses (a) had reached a level in his spiritual ascension where the divine realities manifested themselves before him, allowing the Prophet (a) to hear the voice of God which emanated from the divinely illuminated tree. This miracle itself is a clear sign of the levels of manifestation. Nevertheless, because the degree to which a person can witness divine manifestation is to the same extent in which they’ve lost themselves in the divine, Prophet Moses (a) – who had not completely annihilated his self in the divine – did not observe the complete manifestation of God before him. However, an individual who did reach this level of spirituality was the Prophet Muhammad (s); for he had completely eradicated his self and engaged himself with the love of the divine.
In summary, reaching the necessary degrees of manifestation is a prerequisite to witnessing the divine beauty of God who, without this perquisite, cannot be perceived by the senses. Yet after these levels of spirituality have been achieved, the manifestation of God can be mirrored in the senses. The reason why we say “mirrored” is because a mirror exists in one sense and does not in another. After all, the mirror itself does not exist in essence but the image reflected within it does. Thus, it can be said that the manifestation of God is neither God Himself nor other than Him. Why–because the one being manifested is indeed God, who can be perceived by the mystic that has dispensed of his worldly vision in exchange for eyes and ears which sense nothing but God. In a Sacred Narration (Hadith al-Qudsi) God, the Almighty says this of these dedicated servants: “I will be their eye by which they see and I will be their ear by which they hear.”
 The Holy Qur’aan, Chapter Taa-Haa, Verse 20 “إِنِّي أَنَا رَبُّكَ”
 “[بصائر الدرجات] أَحْمَدُ بْنُ مُحَمَّدٍ السَّيَّارِيُّ عَنْ عُبَيْدِ بْنِ أَبِي عَبْدِ اللَّهِ الْفَارِسِيِّ وَ غَيْرِهِ رَفَعُوهُ إِلَى أَبِي عَبْدِ اللَّهِ عَلَيْهِ السَّلَامُ قَالَ إِنَّ الْكَرُوبِيِّينَ قَوْمٌ مِنْ شِيعَتِنَا مِنَ الْخَلْقِ الْأَوَّلِ جَعَلَهُمُ اللَّهُ خَلْفَ الْعَرْشِ لَوْ قُسِمَ نُورُ وَاحِدٍ مِنْهُمْ عَلَى أَهْلِ الْأَرْضِ لَكَفَاهُمْ ثُمَّ قَالَ إِنَّ مُوسَى عَلَيْهِ السَّلَامُ لَمَّا أَنْ سَأَلَ رَبَّهُ مَا سَأَلَ أَمَرَ وَاحِداً مِنَ الْكَرُوبِيِّينَ فَتَجَلَّى لِلْجَبَلِ فَجَعَلَهُ دَكّاً”, Majlisi, Bihaar al-Anwaar, Volume 13, Page 224, Wafaa’ Institute, Beirut, Lebanon.
 “وَ لَمَّا جاءَ مُوسى لِميقاتِنا وَ كَلَّمَهُ رَبُّهُ قالَ رَبِّ أَرِني أَنْظُرْ إِلَيْكَ”, The Holy Qur’aan, Chapter al-A‘raaf, Verse 143.
 “و کلم الله موسی تکلیما” The Holy Qur’aan, Chapter al-Nisaa’, Verse 164.
 “فَاخْلَعْ نَعْلَيْكَ إِنَّكَ بِالْوادِ الْمُقَدَّسِ طُوىً ”, The Holy Qur’aan, Chapter Taa-Haa, Verse 12. In this context, the command “اخْلَعْ نَعْلَيْكَ” (take off your sandals) has been interpreted as liberating oneself from the [material] attachments.
 “مَا نَظَرْتُ إلَي شَيْءٍ إلَّا وَ رَأَيْتُ اللَهَ قَبْلَهُ وَ بَعْدَهُ وَ مَعَهُ”.
 The Holy Qur’aan, al-Hadid, Verse 3.
 “إِذَا أَحْبَبْتُهُ كُنْتُ سَمْعَهُ الَّذِي يَسْمَعُ بِهِ وَ بَصَرَهُ الَّذِي يُبْصِرُ بِهِ”, Kulayni, al-Kaafi, Volume 2, Page 352, Daar al-Kitaab al-Islamiyyah.