I love solving problems--it is just hard to find the time to work on the little ones. I have been wiring avionics for more than three decades, and I have wired audio panels in both stereo and mono more times than I can count. I've used just about every headset you can think of, and I have used stereo headsets in mono airplanes, and mono headsets in stereo ships. It usually works out, but sometimes, you just have this little problem that won't go away.
Such is an issue I have had on and off with the helmet I use for test flying. It's an HGU-55/P, but rewired with an Oregon Aero liner and headset kit to civilian standards. It is, essentially, a mono headset, with a fairly short cord. This short cord works in some airplanes, but not many--so I have an extension cable with both mic and phone jacks. And in some airplanes, this has worked - but in others, I have had significant issues on the hearing side of things. Since I don't use it that often, the problem has always gotten pushed to the bottom of the stack.
But sitting here on a cold winter afternoon, I decided to see what I could figure out, so I trotted out the helmet and went to the hangar. One of our airplanes is old enough to be wired mono - the others all have stereo audio panels and wiring harnesses. I tried the helmet in all three, and sure enough - it was fine in the mono ship, but I couldn't reliably hear in the stereo planes. Pulling the shells off the stereo and mono jacks, and looking at where contacts were being made showed me why. The mono plug from the helmet was actually shorting the ground and one of the two channels together, while making contact with the hot lead of the remaining channel. The audio panels were NOT happy.
The solution? Well... I found it on the internet, from some obscure post on a message board from long ago. When using a mono plug in a stereo system, you can push it only part way in, and it will work perfectly - which it does! Rather than depend on doing this in each airplane, regardless of jack location, I simply made the "partial connection" where the helmet plugs in to the extension cord - and limited it by using a piece of heat shrink cut to 1/4" and slipped over the plug. It is secure in that position, but will still release if I had to quickly depart the aircraft - the reason for wearing a helmet in the first place.
Live and learn - and solve the little puzzles as they come up!