I have just been in flash audio hell for a few days, trying to get 14 audio files to sync properly and loop in two different fla's
I first has gaps at the end of the loops so they stumbled as they looped even though the audio file was looped correctly. The audio file is exactly 13 seconds and my fla is set to 24fps so thats 312 frames but when you place the loop it appear to be about 1/2 a frame too long. The end result was either gaps, skips or general failure for the loops to "loop".
The biggest thing was don't import mp3's, they have silence at the front that messes up your loop, import wav or aiff files and let flash compress the audio. It may be possible to create looped mp3's and import them but I haven't tried it.
The other thing if you want them to loop, don't let flash do it using the timeline, no "gotoandplay(1)" code it will never loop properly. In the layer properties, if you use "stream" or "start" one loop will stay in sync but any other loops will start to drift away from each other.
Set all audio loop layers to Sync :Event and Repeat 9999. and if you really must, add some code that counts to 9999 and start it again, but at that "loop" it will skip :)
I also found Kenny Bewllews site VERY helpful.