For music, the place for discussion OF music (rather than about making music) is /r/letstalkmusic, which is actually one of the highest quality discussion subs I know of. It's got a relatively slow but incredibly high quality stream of discussions, of all aspects of music history, various bands, philosophy and psychology surrounding music, etc.

For music discovery, /r/listentothis is alright (although it has declined greatly since becoming a default). /r/listentous is sometimes much better and sometimes much worse, by the nature of the sub (elected recommenders). The real best places for discovery are the genre-specific subs however, i.e. /r/metal, /r/futurebeats, etc.