Sounds like a capacitor problem which would come as no surprise.
Check the muting signal on the base of 2TR100 and 2TR200 (or anode of 2D1).
A rising DC here will cause a gradually lower and lower volume and eventually complete mute.
A voltage ramp like this could suggest a leaking capacitor.
If the muting voltage is indeed creeping up, I would check 1C3 (220uF) and 1C15 (1000uF) first, I think.
Still no luck ? Maybe one of the diodes in the muting gate (2D1-4) has developed a leak or the connections
to board module 3 is not the best.
1C1 and 1C2 filters the Dolby supply and they can mess things up too but if the problem affects both
channels simultaneously, unless both exactly equally bad, I suppose we can rule these out since they work with each their channel.
Martin