A cast will not start

When a cast cannot start, the extension says why above the Cast button, with a short heading and a sentence or two under it. Find the heading here.

The tab and the browser

Nothing here to cast: “There is no tab here that can be cast.” Switch to the tab that is playing, then open the extension and press Cast from there.
Your browser kept this tab’s audio: your browser refused to hand over the tab’s audio. Sometimes it gives a reason, and the extension quotes it. Try another tab, or reload this one and try again.
The cast did not start: “Trying again costs nothing and sometimes works.” Press Cast again. If it keeps happening, restart your browser.
The listening part did not answer: the part of the extension that captures the audio did not respond. Press Cast again; if that fails, turn the extension off and on in your browser’s extensions page.

How many, and where

10 casts already running: 10 casts at once is the limit, counting other people’s casts through the same companion. Stop one first.
Still casting a tab: browser-wide capture cannot run alongside a tab cast. Stop that cast first.
Still casting the whole browser: a tab cannot be cast while the whole browser is. Stop that cast first.

The extension also says, before you press Cast, what will happen to rooms that are busy: “Whatever the room is playing will stop” for a room playing something else or cast to from another browser or computer, and “The cast already in this room will end” for one you are casting to from another tab. That is a warning, not an error: the newer cast gets the room. See Cast to several rooms.

The speakers

No speaker began playing: “The audio was offered and no speaker began playing it.” See Speakers found, but no sound.

The audio format

No codec was found: Thaumic Cast could not find out which codecs your browser can encode. Press Cast again, or reload the extension’s Settings page.
96000 Hz cannot be sent (or another rate): if it names your audio device, set the device to 48 kHz or 44.1 kHz in your system’s sound settings. If it names the tab, in the extension’s Settings, under Audio, set Quality to Bespoke and choose a codec other than PCM.

If Settings says “This browser will not encode that combination of codec, bitrate, sample rate and channels, so a tab cast set to it will not start”, change one of the four, or choose a preset instead of Bespoke. The presets and fields are in Advanced audio settings.

Browser-wide capture is on, and cannot be used

If Browser-wide capture is ticked and the companion cannot do it, Settings says “While this is ticked, a cast will not start. Untick it to cast from a tab instead.” It needs the desktop app on this same computer, on Windows 11. See Browser-wide capture.

A cast that stopped by itself

A cast that ends on its own says why in the popup (or, if the popup was shut, in the line beginning “While you were away”). The common ones:

“… started playing something else, and it only plays one thing at a time.” Someone played something on that room from the Sonos app. Cast again when they are done.
“Another cast was started on …, and the newer one gets it.” Another tab, or another computer, cast to that room.
“The cast was stopped at the companion’s end, not from here.” It was stopped from the companion’s side: Stop all casts in the desktop app, for example.
“Audio capture failed partway…” Cast again.
“This tab’s audio arrived at … Hz, and the cast had already declared … Hz.” Cast again while the tab is playing.

When several rooms are casting and only one of them stops, the cast carries on in the others, and the extension says which one left.