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.