621TS Sound Problem

[StellarTV]

621 Experts! Having quite a problem on a 621 which I'm working on for a friend. The set has always had intermittent volume problems. Sometimes the volume would be fine, and sometimes it would be low to imperceptible.

I'm not very familiar with the sound circuit this TV uses, which is inhibiting my ability to try and fix it. Recently the sound has started to break into whistles and tweets when fed with a fixed tone through the test pattern generator. Tube substitution does not help, and the chassis has been found to be microphonic. Voltage checks in teh tuner and sound I.F. are all on the mark.

Here's a youtube I've made demonstrating the problem, I really appreciate any input.

http://youtu.be/cNx9pm-7ck8