I think I just wrote an answer to a question like this..you should really learn to use the Search thing at the top of the page.
Shortly said, most probably you lack some codecs, or to be more precise, the player engine Amarok uses (Amarok itself won't probably play anything) doesn't have the appropriate codecs installed, again probably due to somekind of copyright/other reasons. Anyway.
1) Open Amarok, open settings, see which engine you are using (I guess it's Xine, but could be something else too)
2) Use your favourite package manager to download the needed codecs for that engine; more information you can search from the web, for example if your engine is Xine, then look from Xine's website -- Gentoo uses portage to install packages (doesn't it?) so if you don't know how to use it, read Gentoo's documentation from their website.
If the engine is Xine as I suppose it is, you need libxine-extracodecs. If it was gstreamer, you'd need some gstreamer-plugins and so on.