Have reviewed this thread and realized we could learn more about the unwanted audio tone
just by you listening to its quality. Can you answer these questions:
Hans has said that he's seen an oscillation caused by pots riding at one end.
At 200 kHz ! You certainly can't hear this, but it apparently caused an audible tone in
the audio chain. This kind might be hard to track down without a 'scope.
Now that you can kill the tone by running at low volume, the DC voltage checks should be
done, especially in the audio chain of op-amps - any deviations from those voltages listed
in the manual could help a great deal to narrow the problem area. Can you beg or borrow
a multimeter and let us know if these DC voltages check out?