You will need the schematics and a logic probe to check all the IO chips and the CPU and PIA chips as well as the audio amp. All of which frequently flake out.
If you set up the diagnostic screens to the sound test and press the up/down button on the coion door, the sounds will cycle through and test each of the data lines on the serial connector between the MPU board and the sound card. From there you need to test the data buffer chips (4049 or 4050 IIRC) if you are seeing the data toggle at the PIA, then test the traces between the PIA and the DAC. If you are seeing these toggle, then you might want to replace the amp chip.
If you lose data along the way, just replace the last chip and see if it gets any further.
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