I have Fedora 34 installed (Linux BLUE-FEDORA34 5.11.15-300.fc34.x86_64). I have a pre-existing hardware issue with my audio where one of my channels is broken. As a workaround, I just downmix the stereo audio to mono audio (i.e. combine the left and right audio source before outputting the same thing to left and right output).
Note: I’ve tried using pactl instead of pacmd and change the regex. While I do get my sink name from the pactl list sinks and grep successfully, the part that fails is loading the “module-remap-sink” module.
I had the same question and from another search result found out that EasyEffects (available from both Fedora repositories and Flathub) can do this. Effects, Add Effect, Stereo Tools, select LR > L+R (Mono Sum L+R)