Quoted from arolden:

I already removed those capacitors as a first step. Unfortunately, no effect.
The odd thing is it seems to be an inconsistent problem. Sound seems to improve after the game is left on for a while. Maybe this is a problem with some of the caps after all. I will replace a few others and see if that makes a difference.

Quoted from arolden:

Yes, I have made some progress on this. I realised that the sound improved after the game was on for 10 minutes or so. That led me to believe that the problem was caused by the cold, and as the components heated up, the sound improved. I tested this with some freeze spray today. I sprayed most of the board components and found that sound quality worsened a lot when spraying U20 (AD1851 converter). When I used a heat gun to heat U20 up, the sound improved.
So I am assuming at this point that U20 has somehow failed. I'm going to try reflowing some solder and replacing the caps associated with U20 and if that doesn't work I'll order a replacement. Will keep his thread updated.