That Opto looks good too - forward voltage according to datasheet is 2.1V … you measured 2.0 ← near enough.
Not to distort what we are pursuing right now; that pin 32 is part of the digital signal created when pushing a key to create a sound. Pins DA1 … DA12 then go to the 12-bit DAC IC42 (BA9221) to create the respective analogue signal. When no key is depressed I would assume all these pins to be 0 Vdc or very close to 0 Vdc representing a digital “0” giving an analogue out of 0 Vac … see below table:
Depending on the bit combination data stream that creates the analogue sound.
NOTE:
It is important to recognise that in a digital system it is near impossible to ascertain signal levels with a DMM because a DMM will attempt to represent an RMS value of the readings … both DC and AC - ie the more "1"s as opposed to "0"s on the respective pin the higher the voltage reading on the DMM. You will need at least an oscilloscope with a speed to display at about 10x clock speed of the device you are looking at.