I'm not quite sure on why you cant find your music collection, perhaps you should look into the options menu of amarok, and see which folders are being searched through for music.
As for the part where you say you have no sound when using youtube, it's probably because of you soundcard, either it's not "full-duplex" OR Ubuntu hasn't found the correct driver and therefore it's not using the most out of your soundcard and not using full-duplex to play multiple sources of sound!
You did install ALSA or are you using ALSA in the kernel?
****EDIT****\ Ok I forgot to mention that allthough it's called full-duplex, you should look into Hardware Mixing for solving your "no sound @ youtube issue". I have this URL for you, which gives you a well enough explanation about hardware mixing, and howto set it up. click here!