Sound dropping out is same issue with mine, reseated all ROMs and was OK for a few games. Have narrowed it down to bad connectors on sound board. When I wiggle them, the sound comes good.

My game starts with full sound and then if I walk away after a game or two and come back I'll have no sound at all. Sometimes the first game has sound and then the secon game I play will have no sound.  Is that similar to what yours does?