Jamicon KM6ZMS (Jamicon KM-6ZMS)
It is supposedly the same as a Protac MX320 (and MX322).
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You may have bad caps or a fried transistor in the sound section.
Also looks like some missing jumpers back there.
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If your Protac has a good BIOS copy that and use it.
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https://www.wimsbios.com/biosupdates/large-hd … port/protac/252

Well, I still cannot find the manual, so it will be hard to set jumpers properly. By the way, I somehow managed to get some sound in Windows - when I played some wave sounds, i could hear couple of first miliseconds, and the rest was cut off. And I could hear midi music in dxdiag (but only with midi mapper, not cmi opl3 mode). After reboot again everything was silent. I will try reinstalling the OS later, but I think this could be a hardware problem.

Oh, and I just noticed that the photo was taken before I set the jumpers for the CPU - now they are set for PII 400. Fortunately there is description for the FSB and multiplier setting on the PCB.

M761 does look like the same board only with the BX chipset.
That should not affect the sound jumpers which should be the same.
So maybe try the sound jumper settings in the M761 manual.
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PcChips is probably the actual builder behind all of these boards.
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I was able to find some info about jumpers, in PCchips manual, but they had different numbers. Anyway there was stated that you should close two pins in order to disable onboard sound and I tried several jumpers, but it didn't work. I also found two '0R' resistors near the sound chip, which were looking like a factory-jumper, first one was enabling VCC power and the second one AVCC power for the sound chip. First I desoldered VCC and the sound chip was still detected (probably it had internal connection from VCC to AVCC). Then I desoldered AVCC jumper and the mainboard was having some problems with booting. So I connected everything back (I just soldered some pieces of a wire, not resistors) and decided to desolder DEVSEL pin from the sound chip, lift it a bit and put some paper tape under it. And it works, the onboard sound is disabled now and I put a ES1868F sound card in ISA slot, which works perfectly in DOS and Windows. I couldn't use it with the onboard sound chip enabled, because some games detected it and the computer was getting frozen. It seems to be a nice mainboard besides that problem, the performance of 440ZX chipset is exactly the same as 440BX when I test it in Sandra 2001.

Congrats!