Get unstuck.
Most issues below have one-line fixes.
Routing won't turn on the first time
macOS needs to grant the one-time System Audio Recording permission. If the permission prompt appeared, approve it in System Settings → Privacy & Security → Screen & System Audio Recording, then toggle routing off and on once more.
My volume keys only change one monitor
They shouldn't. Diaulos makes your left monitor the default output and mirrors its volume and mute to the right one. If the two drift apart, toggle routing off and on to re-sync.
I unplugged a monitor while routing was on
Turn routing off and back on after reconnecting. Diaulos keeps running against the remaining display but doesn't yet re-wire itself automatically when a monitor disappears.
Audio died after macOS audio glitched or a sample-rate change
If the system audio server restarts, Diaulos notices and re-establishes routing by itself within about 15 seconds. If sound hasn't returned after that, toggle routing off and on.
I quit Diaulos and my sound goes… where?
Back to wherever it was before: quitting (or turning routing off) restores your previous default output device. Nothing is left behind.