I've got a DK board where the walk sound doesn't fluctuate. Does anybody know where the issue could be? It looks like c13 and c14 have been replaced (black caps instead of light blue).
I think I know what you're talking about. I watch Bens Classic Arcade channel on Justin.tv and I believe his machine does this. The walk sound is the same pitch at all times...it doesn't go up and down, or fluctuate, like you said.
I wish I could help, and I'm not 100% sure this is an actual issue with a hardware component, but just wanted to comment.
According to http://www.mikesarcade.com/arcade/repairs/dkong.html Q6 is responsible for amplifying the walking sound. If you follow the circuit backwards from there you should hopefully be able to locate the faulty component.