Stereo to mono conversion is a dangerous process. There is a caveat: broadcasters often apply stereo wide expansion effects done by inverting one channel which do not add to mono very well. My all purpose safe solution is to take only the left channel and look in the device configuration for options to switch to mono.

I've lived that first hand. One user connected a cheap wireless headphone to a TV set. The headphones had a switch to set Normal (Stereo) mode or TV (Mono) mode, TV mode had to be used to stop a high pitch noise. The user complained that it worked fine most of the time but her favourite program would sound very faint and turning the volume up would result in very distorted sound. I found that switching the headphones to stereo would cure the problem but it resulted in the high pitched noise back. So that was it, that program was broadcast in wide stereo made by inverting one audio channel and the headphones summing of both channels resulted in almost no sound.
As a curiosity, enabling the TV builtin stereo enhacer did invert the situation: the affected program sounded good and any other had a very faint audio.

My solution: I disassembled the base station, unsoldered the right channel and connected the left channel to both inputs. Not a high tech solution but she was very happy with it.