this extension Just Works; 5.1 audio is properly normalized so that dialogue is actually audible and I can watch 4k 21:9 films on my 21:9 monitor without silly black bars. Thank you so much!!

THIS is the only viable solution for the volume issues in Plex. I am astounded that Plex hasn't incorporated this into their web player. It's really nice to be able to watch HD wide ratio video on an ultrawide monitor without black bars now too.
Works on latest firefox - but I had to enable it to work on all sites in order for it to work properly. Enabling the plugin for the URL of a local Plex server, which is on a 192.168.* address, does not appear to work. That works in Chrome without issues.