Moin, if I connect my monitor with an HDMI cable to my laptop and then connect cellphone headphones (jack) to the monitor, I only hear sounds, but my microphone is not recognized or something.
It doesn't have to work if the monitor doesn't have a microphone input. You should first clarify whether he has something.
It has a jack connection
TVs also have jack sockets. There's also only a sound output and no microphone input.
Put a headphone in there. Then you will hear something. I don't know any monitors with a microphone input