I was getting the sometimes no sound/sometimes crazy sound symptom with a failed 6808 at U11... however mine would always fail as opposed to work intermittently. You can try swapping it with the 6808 at U17. If your game won't boot, its probably the 6808. In my case, even if the game wouldn't boot (the one digit display on the MPU usually showed a code of 7 in my tests), the sound test would still work.

I would expect your sound test to work under this condition (there are two pushbuttons on the MPU, the one on the bottom will play through the sounds). Mine was intermittent, but when I was in this phase of testing, the pin on the connector to my 12v supply was suspect.