If it’s your own music (i.e. you’re playing it) then you can save the file as an mp3, and load it up at musicwebtown.com, and do an [audio] link to your playlist there. You can try submitting new music to last.fm as well – you might pick up some new fans.
If it’s not your music, i.e. it’s commercial music, which is released by a record company, then you’ll run into copyright issues if you follow that path.
Two of the best routes for playing commercial music in WordPress.com are through last.fm and playlist.com, although they work slightly differently. See this explanation for details: