First prepare your sounds in an external WAV editor, like Sound Forge. In order to loop well, a sound needs to be timed correctly. The best way to do this is to compose the tracks yourself and then cut the loops out. I wouldn’t recommend that you record  loops from commercial music, because you might find yourself on the wrong side of copyright law, but if you don’t want to set about composing yourself, you can buy sound loops from the Internet, or look for some public domain ones, (www.loopz.com or www.sonicfoundry.com are good sources). 

Open Flash and import these sounds (File>Import). Make sure that the sounds are in your Library and that you don’t have any sound events set on your main timeline (double-click on a keyframe, and check that the Sound tab displays None). For this example, I’ll call the sounds track1.wav, track2.wav, track3.wav and track4.wav.