Yes, I think that the government must intervene sometimes and has the ability to censor rap. Even though freedom of speech is a protected right, sometimes I think that rap goes too far and aims to talk about adult subjects with very young kids. When that happens the government can then censor rap.
We have freedom of speech in the United States, it is true. However, certain ways of speaking are just not allowed. We are not allowed to yell "fire" in a theater if there is no fire because that causes chaos. We are prohibited from using some slurs against certain groups because it dehumanizes them. If certain phrases and words incite to violence and dehumanizing, that rap needs to be prohibited.
However a nongovernmental rating authority could establish itself just like they did for movies. That would be reasonable.
Freedom of speech is the most important right. I can see restricting speech if someone is explicitly calling for violent acts or telling lies that lead to imminent chaos (i.E. Yelling "fire" in a theater) BUT beyond that any reasoning that "well people listening might be inspired to be violent" opens the door for any censorship the government wants. By that logic the government could decide that any speech talking about "The gap between rich and poor" is illegally inciting violence because people might listen to that and decide to attack and rob rich people or even attempt a communist revolution. And they could also apply that reasoning to conservatives talking about "too much social welfare" and "personal responsibility" reasoning that someone might listen to it and decide to get violent against the homeless.
The person who chooses to be violent is the one who is responsible. Unless speech contains an explicit violent threat or explicitly tells someone else to be violent then it should be allowed (excepting lies that create panic i.e. "shouting fire in a theater").