Sometimes, pulseaudio would assign a stream to a particular sink that a) was not the default sink and b) was not related to prior settings. I wanted pulseaudio to assign all streams, by default, to the default sink.
Apparently pulseaudio remembers where each stream goes, so the next time it sees a stream from that application, it will try to assign it to the sink it was previously assigned to. I don’t want this behavior.
In /etc/pulse/default.pa, replace
load-module module-stream-restore
with
load-module module-stream-restore restore_device=false
and reboot.
Originally found on ArchWiki (like so many other great troubleshooting guides).