Zero asks for the microphone the first time you press Zero on the dock at 0xr.space, with the browser's own dialog. Choose Allow. If the popup was closed, the panel says “The permission popup was closed. Tap the mic again and choose Allow.”, and a Try again button asks once more.
If you chose Block, the browser will not ask again and tapping cannot help: the panel says Microphone access is blocked for this site. Allow the microphone in the browser's settings for 0xr.space, then press Zero. In the Model Explorer app on Meta Quest the headset stops asking after a refusal; tap the mic to open Settings, or go to Quest Settings, Apps, Model Explorer, Permissions, and turn Microphone on.
It may be hidden behind you or to one side. Look around for it, or leave and re-enter the room, then press Zero again.
Another app is holding the microphone. Close it and try again.
Only while its session is open after you press Zero. It ignores speech that is plainly not for it.