Unexpected (unintended) capacitor charging explanation
MUTE should not go below -5V with normal transistors, or else they may suffer reverse base-emitter voltage breakdown.
Which might pull an unloaded output to -2V (with MUTE off) or so... charging the capacitor to give you a surprise when operating MUTE.
Check the datasheet for this transistor (it MIGHT specify a higher voltage), and if necessary, replace them and take steps to reduce the -ve swing on Mute.
EDIT : Datasheet DOES specify Vebo = 25V max, so this ought to not be a problem. This is unusual, perhaps these transistors are designed for this specific role.
But check for substituted parts or possible counterfeit parts, and/or test a few samples for possible out of spec leakage (at say, 10V)