Intermittent low sound volume level...

Hi, new SS owner as of this Christmas. My SS sound mostly starts "normal" (loud, nice volume level), but after some game play, and usually triggered by the bell sounding, drops to a low volume level. Sometimes it comes back to the normal level when the bell sounds again. Once, a static shock jumped from my hand to the shooter knob and restored the sound level. The problem has been steadily getting worse, with more time at the low volume level than the normal level, even after a cold power-up. The speech sound and other sound effects seem otherwise complete and normal. My electronics does not have a sound pot next to the sound test push button. The system hums a little, and is modulated a little by the lights during "attract mode", but does not seem excessive, but the game is not completely silent either. Any ideas or pointers appreciated (EE degrees, not afraid of schematics)! Thanks!

Play tip in case you don't know: if you have locked balls when your last ball drains, press the credit button right after your final score is tallied. Balls stay locked, and new game starts! Sort of like today's video game "insert another token to continue".

Update: Silly me. I found the volume control inside, and after cycling it a bit, the problem is gone.