What happens when you push the sound test button on the left of the MPU board? (Does system 11a have one?)
My only experience with tracking a lost portion of audio on an 11b game, music but no voice, was, in sound test via coil door buttons, music worked but no voice was heard and there was a noticeable hum. Hooked MPU and sound board up on a bench and pushed the sound button on the MPU. Alternates a high beep then a low beep. Only one beep was working. Looking at schematic and using a scope, I traced it to a $1.79 OP amp. Signal was going in, but nothing coming out. Replaced it, hum went away and call outs were back. Owner had the game for years and didn't realize it had call outs.