In foobar2000 it's kinda easy, Preferences -> Playback -> DSP Manager -> Downmix channels to mono [+]. (picture: https://i.imgur.com/FDhxh3j.png)

Thank you for the intel. I'm testing it right now. It looks like it has everything I need, global hotkeys, sane way of sorting playlist and mono.

And... Wait a minute... There is a mobile version... Is the end of the nightmare right here?

edit: You made my day. Thank you very much.

For Linux there are various command-line players, e.g. mpg123, with this option and with any other imaginable options.

I want a full working application with the equivalent of global hotkeys. I use deadbeef on GNU/Linux, but I still haven't find a way to force mono.

Out of curiosity, why do you want mono? Just a preference?

Yes. And especially when listening with headphone, some artists find it fun to have a moving song from one side to the other, in that case I turn my head thinking that there is something near me, absolutely annoying. With mono the sound is always located in the middle of my brain.

Ah, so you essentially take the existing stereo mix -> force it to mono -> play the new mono feed in both ears? That's an interesting idea...

I feel like this should be relatively easy to replicate with VAC (https://vb-audio.com/Cable/) and Audacity, if the player doesn't support it natively.

Why would I need to do that? Winamp does it perfectly.

Beatles is the reason.