I know that the answer is long too late, but it doesn't matter who else will find it useful because headphones are a horror for every user. Either something is not connected or the cables break or break off.
I'm no expert, but it seems to me that each microphone is monophonic. In most cases, the signal from the micro in headphones for computers is separated by a bundle of cables into the left and right channels, or it is done only on the plug. Of course, if it is a mono microphone, then mono signals can be passed through a special mixer and then the sound is pseudo stereo, as in the case of, for example, Team Speak 3 (you transmit mono and they hear you in stereo).
But it does not matter.
You can take a look at your headphones. What is the plug with micro (stereo or mono). If it is stereo and you only have sound in one earphone and you are sure that the mixing program on your computer is working correctly, then it is worth checking how it is connected with a cable inside because the microphone transmits a mono signal ...
I once bought a Saitek Pro Flight Headset that worked fine in both channels when I used the microphone. I mean, when I had the microphone listening turned on to hear my voice in the system, it reached the right and left earbuds and I didn't have to combine software like my colleague above. But the cable was worn in one place and I had to replace it with a new one, because unfortunately the quality of the cable was poor and even repairing and playing with soldering and shortening the cable gave a short-term effect. When I wanted to replace the cable with a better quality, one that has everything in one, during the exchange I came across a problem with the microphone.
After a quick replacement of the cable and connecting to the computer, only the sound from the microphone could be heard in only one earphone, which surprised me ... I didn't want to modify the plug, because it would look ugly later (one original, the other modified). So I bought 2 stereo cables and used the microphone and headphone connections separately. The thing about the microphone is that the cable cannot be patched. It must consist of a single piece. From plug to microphone connection. Any tampering with soldering, cable connections or a poorly insulated plug has an impact on quality. If we use the whole original cable then additional interference can be avoided.
In order to have sound in both headphones from the microphone, you need to allow the signal from the micro to be transmitted to the computer on the left and right channels. That's like stereo. So I connected the microphone by jumpering the wires from the right and left channels. (for the lazy, a similar jumper can be made on the plug by connecting the cable to the Left and Right simultaneously). After connecting to the computer, then the sound from the micro sounded in both headphones, and this is how the microphone should work. To this day I use these headphones and the sound is clear without interference, so sometimes it's worth knowing how what works and how it should be connected.
Greetings.
