I know your probably all think JUST GET MUSICPLAYER, but i don't have that many mp3s on my hard drive

Anyway I'd like a mod that can integrate playlist.com playlists into the game like musicplayer does, except from the site.  I have the xFire in game web browser and can already listen to the music, but i'd like reverb and voice chat muffling support in with it, plus easier switching between in game music and the playlist.  

Addons cannot send or receive information via the internet except through WoW, and the base UI does not allow what you want. The only work around is if the site you want makes a program that puts an overlay in game, as XFire does. You may want to contact playlist.com and make a suggestion.

like I said about xfire though, I can't reverb, and it makes voice chat hard

Get a second sound card and a pair of speakers. Fixt.