Thanks to everyone who tried to help.

Somehow, YouTube video playback suddenly stopped working. While searching for a solution, I stumbled upon this thread: https://bbs.archlinux.org/viewtopic.php?id=276918 , and found that running

pacman -Syu pipewire-media-session

did the trick.

Not only did YouTube start working again, but I also started hearing sound from my device — the volume meter began responding too.