Playing MP3 and video files is not a difficult job in Linux. and I think some one must have already answered the same question before. Try giving it a little time searchin for the similar thread, and you'll find answers. But as a hint, you can play mp3s using XMMS, a real good player for playing music files and Videos can ofcourse be played using Xine, Mplayer. You can find more info about them on google.
Try it out and n'joying the great music playing capabilities of Linux.