If you’re wanting to install Exaile on your Gentoo box AND have it play MP3s (and perhaps this goes for all multimedia applications), you’ll want to enable the mad USE flag.

Oh, and so far I’d recommend Exaile if you’re looking for a player that doesn’t come with a lot of baggage — like Gnome/KDE libraries. It’s fitting into my XFCE4 desktop very nicely and has all of the features you’d (or at least, I’d) expect from a player.