Set HomePod mini as default speaker instead of built-in

Pretty much, yes. macOS doesn’t have the same permanent HomePod default setting that Apple TV does.

You can select the HomePod mini from Control Center > Sound whenever needed, but the Mac may switch back after a restart, sleep, or an AirPlay disconnect. A wired or USB speaker is more reliable if you need a permanent default output.

It depends on which device keeps switching back to its built-in speakers.

With an Apple TV 4K, you can make the HomePod mini permanent: go to Settings > Video and Audio > Audio Output, then select the HomePod mini under Default Audio Output. The Apple TV and HomePod need to be assigned to the same room in the Home app.

On a Mac, iPhone, or iPad, the HomePod mini is selected through AirPlay, but there isn’t a setting that permanently replaces the built-in speaker for all audio.

Which device are you playing the audio from?

You are not out of luck, you just need to re select the HomePod as audio via settings >sound if the Mac forgets them. But if you want to have the output automatically to the HomePod, it seems like that you're out of luck

Thanks for confirming. That matches the limitation here - HomePods work as an AirPlay output on a Mac, but macOS doesn’t treat them like a permanent default speaker.

If you have the Sound displayed in the Menubar, you don't have to spend time opening System Settings to reselect the HomePods.

Yes, this is my experience as well, using a stereo pair of HomePod minis as the audio output for my M1 Mac mini.

Not only does it revert to the internal speaker after sleep or a restart, but it also happens randomly even while they are being used. There is no rhyme or reason. ¯\_(ツ)_/¯