Do a search for Flash MP3 Player and I'm sure you'll get oodles of results. The easiest way is to create Sound objects, and load .mp3's instead of having them weigh down your file by attaching from the Library:
myBeat1 = new Sound();
Make buttons to start or stop them, like:
and you'll probably need to control the sound volume with slider(s) or buttons(s) - just check some more polished examples out...