You're going to need use a DMM to test some voltage values on the CPU. This has to do with your sound issues ONLY.
Set the meter to DC Volts and auto range. On the upper part of the CPU are the areas you'll need to test. Put the black lead on a known ground or test point that indicates GND.
Using the red lead, directly below U34, check for +12VDC. The test point is labeled +12V. Moving to the right about 3 inches, check for +3.3VDC and +5VDC where indicated. About 2" above the +5V test point is the -12V test point. Unlike B/W WPC driver boards, the test points aren't labeled as such and are very small.
Mark down the values you get and report back.
Unfortunately, most of the Stern PCB's components are surface mounted and if a problem exists that complicates matters.
As for the opto issue, start by following the wires back to the opto board and make sure that they're properly seated.