I reinstalled Ubuntu Studio recently, excised all Snaps, and installed Firefox from Flatpak. Afterward, I didn’t have any audio output in Firefox. Videos would just freeze.
I don’t know how much of this is fully necessary, but I quit Firefox, installed more PipeWire pieces, and maybe signed out and/or rebooted.
sudo apt install pipewire-audio pipewire-alsa
pipewire-pulse packages were already installed.
AIUI, this means that “PipeWire exclusively owns the audio hardware” and provides ALSA, JACK, Pulse, and PipeWire interfaces into it.
It’s not perfect. Thunderbird (also flatpak; I’d rather have “some” sandbox than “none”) makes a bit of cacophony when emails come in, but at least there’s sound for Firefox.