I still say that the original problem of only Flight Leads hearing the .wav file (while odd in itself) might be better served by using the 2 step method of having the area event set a flag, and a time event using that flag to trigger the message and .wav file.
There will still be a problem with FLIGHT A and FLIGHT B as AREA EVENTS are geared to the PLAYER I don't THINK they work with other Human flights in Multiplayer, only the PLAYER in BRIEFING GROUP A. In this case, the better option might be the WP ACTION FLAG SETTING.