I noticed that the labels for the per-channel volume controls in “Sound - System Settings” seems to be swapped in relation to the test buttons.
“Rear Left” volume bar changes the volume on the “Front Center” speaker, and “Rear Right” changes the volume for the “Subwoofer”.
Looking at alsamixer, it seems that the problem is in the test buttons and not on the volume controls.
I am not sure if this can be related to my hardware and I don’t have much experience reporting bugs, so I decided to post here first.
I am running Debian Unstable with Plasma 6.3.6.