Flash audio dropping in FLV playback
From Birnam Designs Wiki
The sound will drop if you are using multiple FLVs in a single FLVPlayback control and switch the active or visible video. To get it back, drop the volume down to 0 before the flip, then back up to the previous volume after switching. Example:
var prevol:Number = vid.volume;
vid.volume = 0;
vid.stop();
vid.activeVideoPlayerIndex = 1;
vid.visibleVideoPlayerIndex = 1;
vid.visible = true;
vid.play();
vid.volume = prevol;