How to loop sound

I want to add a music loop (8 beats) to my movie, whats the best way to loop a sound seamlessly?

It’s not clear if you mean the actionscript of the audio edit. If you mean the AS,

myLopp = new Sound();

myLopp = name of your sound object.
myLopp01 = linkage identifier of sound object
999 = number of loops
0 = offset