Here's the solution I use.
I bought a really cheap GoPro - like camera (£16). Video is pretty poor but I wouldn't be too upset at that price if I ruined it. I took it apart. There's a mic on the printed circuit board. I unsoldered this and soldered 2 wires to a 3.5mm mono jack I installed in the case. Next, I got a 1/4 inch headset plug and installed 2 resistors in the plug (to drop down the voltage from the headset), then wired a 3.5mm mono plug on a lead to this 1/4" plug. I then made up a headset splitter, so you can use the arrangement with a regular headset. The whole thing came to less than £20 and an evening's work. I use the camera on a mount facing the P1 so I can get lip sync with the other better camera in the cockpit. It works great. The earlier versions of GoPro do have a mic input but I think it's a mini USB (NOT micro!). Not sure about the latest Hero 5 (beyond the budget of a flying instructor!)
TOO