[morzh]

Logic dictates that if your volume regulation's off the first thing you should do is check your volume regulator potentiometer.

Desolder your #44 (I think) volume regulator and check its operation. See how the resistance between the wiper and the top / bottom ends of the resistive scale changes when you rotate it.
Also see the end-to-end resistance.

The resistors with a tap sometimes develop cracks and the end-to-end resistance becomes infinite or too large.

See how it compares with the nominal value.

Then we'll talk further.