Music is essentially sound, and watching it (without listening) has no ruling; the only ruling there is, has to do with producing and the sale of instruments, teaching and learning, playing, and listening to it. Most scholars consider listening to and producing lahwi music to be haram and don’t see all music to fall under this category, therefore, it can't be said that all music is haram. [i]
[i] Adopted from Question 10803 (website: 10622) .
“Music” is originally a Greek word which is synonymous to “Ghina” in dictionaries, but in fiqhi terms, there is a difference between the two. Ghina refers to the singing voice in which comes from the larynx and is turned up and down in the throat [like the chirping of birds] that somewhat ecstasizes the listener and brings him joy and is suitable for gatherings of amusement and vanity. As for music, it refers to the sound made by musical instruments. On this basis, it is correct to say that not all things considered music scientifically are considered music according to fiqh  . 
Watching music (without listening) has no ruling; the only ruling there is, has to do with the production and sale of instruments, teaching and learning, playing, and listening to it. The grand Ayatullahs have issued their fatwas on the different types of music:
Hazrat Grand Ayatollah Khamenei:
Any type of mutrib music which befits carouse gatherings is haram .
Hazrat Grand Ayatollah Fazel Lankarani:
Any type of mutrib music which befits carouse gatherings is haram. In any other case then there is no problem, and the place from which it is broadcasted does not affect the ruling.
Hazrat Grand Ayatollah Behjat:
If music is mutrib then hearing and buying or selling it is haram.
Hazrat Grand Ayatollah
That which the common culture recognizes as music is haram, and listening, composing, teaching, learning, and selling musical instruments is also haram.
Hazrat Grand Ayatollah Makarim Shirazi:
Any melodies or songs which befit libidinous pleasure and corruption are haram, and anything other than that is halal. This is distinguished by referring to the public common culture.
Hazrat Grand Ayatollah Tabrizi:
Listening to music which befits gatherings of revelry is not permissible. It is also not permissible to make, teach, buy or sell frivolous musical instruments.
Hazrat Grand Ayatollah Sistani:
If music does not befit frivolous gatherings, then it is not haram. Therefore, listening to songs and mutrib music which befits frivolous gatherings is haram.
Distinguishing whether or not a specific music such as rap music is a type of instance or applicability of haram or halal, is the responsibility of each person. Because, any individuals that have a even a concise understanding of music can differentiate between music that is mutrib and befits frivolous gatherings and music that is not like that. Therefore, even if revolutionary or war songs are mutrib and befit frivolous gathering then listening to that type of music will also be haram.
Rap music is not exempt from this rule, and if it were to fall under the category of mutrib and befitting carouse gatherings then it also would be haram. 
For further information, see:
1- The reasons for music being haram or halal, Question 6964 (website: 7088).
2- How to distinguish between haram and halal music, Question 392 (website: 408).
3- The fitrah and the impermissibility of music, Question 10803 (website: 10622).
4- The reasons for the impermissibility of music, Question 10868 (website: 10755).
 Seyyed Mojtaba Hoseini, Porsesha va Pasokha, pg. 169; Imam Khomeini, Al-Makasebul-Muharramah, vol. 1, pp. 198-224; Ali Hoseini, Al-Musiqi, pp.16-17; Tabrizi, Istifta’at, questions 10, 46, 47 and 1048; Fazel, Jame’ul-Masa’il, vol. 1, issues 974, 978 and 979.
 Adopted from Question 6964 (website: 7088).
 Adopted from Question 392 (website: 408).