One aspect:
A good chunk of vocal music today – be it rap, pop, or something else – has some rather empty, negative, or contrary-to-Torah values. Rap music is certainly (no statistics here – I don’t think it’s necessary) of the more explicitly risque/vulgar variety which I think conjures the “red-line” much more.
Another aspect:
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We are influenced by what we hear or see and all that comes with it whether we are aware of it or not. If one spends time around the street, they come to look more and more like it regardless of if it is conscious or not. That applies both to vocal or non-vocal music.
With regards to Mozart: I don’t think there are cultural norms for people who listen to or are even involved in classical/orchestral/symphonic music.
Assur? I’m sure fingers can be pointed to things like “bechukoseihem” and the ilk, but I think it can be useful to start with the a seicheldige understanding.