Quoted from wiredoug:please .. no .. dont cover the board in contact spray. The pot at R8 is not a volume control its a balance control.. one side plays sound, the other speech, the middle a suitable ( to the owners ear) mixture.
Turning it up doesnt make the sound come back it just increases its share of the mix .. put it half way.
Im not saying cover the board. Im saying squirt a tiny bit on the pot. Or ruubbing alcohol. Twist it back and forth to clear any dead spots.
Then set it back to the middle. Or where it sounds best.
If/when it gets a dead spot or goes bad. You will lose sounds or hear them faint. Either clean it. Or replace it first.
Its a cheapbsimple fix. If it stll doesnt work after that then look into it further.
1 step at a time, simplest solution to hardest.
Dont work backwards.