Plex-desktop when installed from snap in Arch Linux, volume control is inconsequential, meaning your roll it up and down and does nothing to the volume.
same on Ubuntu 22.04
This is also mentioned in plex forums but still not fixed https://forums.plex.tv/t/plex-desktop-1-59-1-audio-volume-slider-no-longer-working/822150 As a workaround I refreshed snap using latest/edge. 1.47 version. sudo snap refresh --edge plex-desktop