Have you opened a .swf file recently?
ive had this issue with this player before, i open a swf file and i just close the player, then next time i try to open a video file it is completely mute,
what i have to do is open a .swf file again,
then once the player loads with the file you will here sound from it,
now that you here the sound you need to load a file by pressing Ctrl+O
this SHOULD bring back the sound,
this will only work if the sound went away AFTER you had opened flash file, if you here sound when you load a flash file i think this will also work