Make a new movie clip and place your sound into it. Then place an instance of the clip onto the stage. The sound MC will loop round wether the main movie is playing or stopped
open the sound panel.
in the timeline, select the frame where you want the sound to start.
in the sound panel, select the sound you want to put(a sound already imported in flash).
then you can select 4 different sync: start,stop,event and stream.