Thanks for all the ideas folks. The strip around the outside of the case is actually a silver/grey rubber material... not metal. I've tested it with a continuity probe and it's not conductive. That's not to say there isn't a metal strip inside it though... so I'll try taking it off this weekend to see if that makes any difference.
Thierry - thanks for the tip on the foil. I was merely copying how the circuit board was mounted inside its original Etherwave case, but I'll remove this to see what happens.
Fred - The volume circuit seems to be just fine, and luckily it isn't affecting the pitch that I can tell.
I likely won't get time to try again until this weekend, but I'll keep you all posted!