All terrific info. It's too bad there are no schematics and part numbers.
I guess I will replace the large capacitors that are near the volume pot. If that doesn't clear up the audio, I will just leave the speaker unplugged.
The only other thing I would like to have is how to use the test feature.
Other than that I will have to keep ant eye out for a repair manual.
That motorboating sound problem can also be caused by the power supply, it has many capacitors to keep electrical circuits in the machine from sending unwanted feedback (and other interferences) from one circuit to another. Since the power supply connects to everything electrical this can happen. For example, if one of the power supply capacitors fails then you can get feedback from the sound output stage that goes back into an earlier sound circuit and gives a sound problem.
If you can get some spare parts for your machine at a reasonable cost I think it would a good idea, good working parts for these Sigma machines are not as readily available as they used to be. Spare parts like a cpu board and power supply would be good to have on hand for troubleshooting and in case of a failure at some time in the future.