Dell Inspiron desktop, when I plug headphones into the front-panel jack, the headphones work, but it does not mute the speakers which are plugged into the back-panel. Instead, the speaker volume reduces by about half, but still clearly audible.
This behavior started some months back. Can’t say exactly when, because I don’t often use the headphones. It definitely used to work.
Not hardware related, since it works perfectly on Windows-10 on the same machine (dual-boot).
Where to look?
I propose you to test the live image of Fedora Linux 34 and see if this is solved. F34 comes with greate changes about Audio.
Good suggestion, I tried that. Unfortunately, I was unable to run the Fedora-34-Beta-KDE, since it hung my machine. I filed a bug report Bug 1951695.
Just to confirm my own sanity, I went back and ran a live USB of 33 KDE Spin. I confirmed there that plugging in the headphones correctly muted the speakers. This confirms that it was some software update after the initial release that caused the problem.