I'm a member of the local search and rescue patrol, and as such I use portable radios when out on missions. I wanted to get an earpiece so I could listen to radio traffic. Of course, I could have bought one but they're like $40 each and why buy when you can make a reasonable replica for a fraction of the price?
This picture is the result. The process turned out to be pretty simple. The microphone already has a 2.5mm jack. The big question I had was what type of jack it was. Turns out to be just a mono, listen-only jack; there are no microphone leads. So coupling a standard pair of headphones to a 3.5mm stereo male to 2.5mm mono female adapter was all it took. The one caveat there is that only the left earphone will receive any audio, but that's exactly what I wanted. A few zip ties later and the whole thing was ready to go.
First problem was the thing is loud. I don't know if that's just my radio (Tait TP9100), or my headphones, or what. I suspect the expensive earpieces attenuate the signal significantly. There's also a lot of static which makes conversations difficult to follow. Luckily for me, the speaker on the microphone has a Hi/Lo switch which solved the problem.
Given what I paid (about $2 for the adapter), I'm quite satisfied.