I would like to inform you that down sampling stereo sound to mono sound is not possible since stereo sound is a standardized sound setting.
Stereo speakers are designed to produce sound on both the speakers and there is not option to change the settings to use it as mono sound.
Of course it is possible, you redirect the left and right audio channels to the same destination, thus the left & right audio channels are audible from each individual speaker instead of only the left audio audible from the left speaker and the Right audio
audible from the right speaker.
So both your left speaker and the right speaker produce identical audio to each other. It's pretty dumbfounding that you think there is a technical limitation in place that prevents it from occurring when in actuality the only thing that can prohibit it
is artificial limitations.
Edti: Also a lot of the suggestions so far with regards to setting the Audio Playback Device to mono or enabling "Headphone Virtualization" or something else etc. is entirely dependent on the sound device being used and the drivers that are installed for
it.