Using a standard PC gaming / smartphone headset for passengers in a small plane
This is an interesting question. But, I don’t see what this solves. There are aviation headsets as cheap as $50. That is as cheap as gaming headsets. But, they are specifically designed for aviation to include passive noise reduction. There would be no advantage to using a gaming headset in aircraft.
The primary purpose of aviation headsets for the pilot is to provide clearer communication between the pilot and ATC. But, that purpose shares equal importance with the goal of hearing protection.
Small, General Aviation aircraft are very loud. But, aircraft have speakers and microphone handsets that make communication with ATC still possible. In decades past, headsets were not even worn in enclosed cabins. And, it has only been recently that the effects of noise has been considered a factor in fatigue. Unless the headset addresses the concern of noise reduction, it is of little benefit in the aircraft.
For passengers, noise reduction can be accomplished with industrial or firearm hearing protection. Non-crew member military personnel function with only standard earplugs aboard aircraft. Communication between personnel in the aircraft even with the doors open is accomplished through shouting over the ambient noise. As far as the pilot turning off a Bluetooth device to enforce “sterile cockpit” practices, that can already be accomplished as a built in function of the aircraft audio panel.
So, although a very simple adapter could be built to enable gaming headsets, cellphone headsets, and earbuds to be used in aircraft, it does not truly solve the problem you posed so far. And, it is not truly better than purpose-built aviation headsets. However, my purpose-built aviation headset is Bluetooth enabled, and can be used with a Bluetooth enabled computer like a gaming headset.