Weird issue using HDMI

jg's icon

In my work, we have some new large-screen Sony Bravia TVs we use for teaching: I connect my laptop to the TV via HDMI. The laptop is an M1 Macbook Air running the latest OS; MaxMSP version is 8.3.0.
- if Max's audio out is set to be the laptop internal speakers while the image is shown on the TV, both image and audio work fine
- if I set Max's audio out to be the Sony TV via HDMI, I get no sound from Max. Even stranger, it seems that Max stops processing audio altogether: I see no activity in meters within the patch or the meter in the toolbar (bottom right of the patcher window). Max doesn't crash; the UI contines to work as normal (except for the lack of meter activity); there are no error messages in the Console. If I switch back to the laptop speakers, the patch runs normally.
- other audio apps (like Apple Music néé iTunes) work fine via the HDMI connection.

I've searched the forums, but no-one else seems to be reporting this issue. Has anyone had anything even vaguely similar? Or have a suggestion for something I might check?

Thanks,

J