Sorry, I don't have a schematic but all these chips are located very close to the volume pots. I believe the two ROMs (with labels) close to the pots are sound ROMs. If ROM test passes, they're probably good, but if you had an EPROM reader, that would verify the code in them.
The Z80 CPU for sound is also in the vicinity of the volume pots, but I am not sure which of the three Z80s on this PCB it is. Without a logic probe, there is little to do except check for voltages on the GND and 5V (you can get the pinouts through a google search). If the CPUs on this PCB are socketed, you could exchange with known working Z80s and see if the problem persists. As a matter of fact, and again if the chips are socketed, you can move those three Z80s around the same PCB and see if other problems arise. For example, if the sound Z80 has issues and you place it in the socket for the game code running Z80, the game would probably not work. That would confirm a bad Z80.
The sound chips YMxxx are also in the vicinity of the volume pots. The pinouts are out there, you can check for voltages, but again, without a logic probe, it is hard to know what is going on.
The problem with this PCB might be simple, but figuring out what it is may involve time and some additional equipment purchases. At this stage, I would think about that and how comfortable you are in terms of devoting time, effort and money. It may be easier to either send this for repair or getting a known working one... this is a "fairly" easy to find PCB so perhaps the repair costs would be more than a completely new purchase. There are threads on who repairs what on the front page of the this forum category.
If, on the other hand, you are interested in repairs, then the extra equipment purchases may not be a bad idea...
My 2c..., sorry I could not be of any other help...
paris