Closeup of a PCB. There's two adjustable volume controls on the left, several Yamaha chips, and a press-fit connector labeled Mini-ISA/GPIO
https://digipres.club/system/media_attachments/files/112/215/141/021/566/320/original/128a9ea1a86caf95.jpeg
Under those panels:
The top-left one shows the OPL3 chips, and it has trimpots for volume (left and right?)
There's also a Mini-ISA/GPI socket, which I think is documented somewhere
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