Like yesterday I did it on KDE Plasma on Neon , now I also do it on GNOME 3 on Debian 10. If you have two speakers on left and right, it is very easy to adjust the volume independently by using built-in System Settings in the Sound section. Simply slide the Balance slider to left or right. For instance, I adjusted it to right (as my left one is currently broken) so I will listen to sound from the right speaker only. And vice versa. That's it.
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