I use
AMPlifier straight out of 3.9 to play .mp3's through my AD516 sound card. Works fine for me and very nice sound quality. I'm sure it could also play through Paula but I haven't tried.
There's also
AmigaAMP, which I understand is a bit newer and has extra plugins and skins, but for me it seemed a little more "stuttery", like it needed a faster processor than my '040. Maybe I just didn't mess around with the settings very much.
If you have a lower-spec system check out the
MAS-Player.
Or if you don't hold your breath waiting around for it, the
Prisma Megamix (announced two years ago, still not available) might be released sometime this century.
There's a few other soundcards and accessories available for playing .mp3's, check the BBOAH or amiga.resource.cx website.
