Why couldn't you make a gameevent with FLAGTAKEN pointing to a conditional relay that is FLAGONSTAND targetting the speaker and then 2 more game events targetting the speaker with FLAGRETURNED and FLAGCAPTURED.
That should allow the gameevent ONLY to fire the speaker ONLY when the flag is taken from a flag stand. Then it should shut off when the flag is returned or captured.
Did you try that and it not work? :(