U3223QE, SB522A, 2019 MacBook Pro Sonoma, no sound

I have a 2019 (Intel) MacBook Pro running Sonoma 14.0, plugged into a Dell U3223QE with a matching SB522A slim conferencing Soundbar. The Mac is plugged in to the monitor using a Thunderbolt cable, plugged in to the USB-C/PD/DP. The Soundbar is identified by the Mac as a USB audio device for both input and output. No sound comes out of it at all and the microphone doesn't appear to work. The light on the left comes on briefly when I plug the Mac in but doesn't stay on (I don't know if it should).

Some further clues:

• the Soundbar works fine with a Linux machine on an old HP Spectre x360, plugged into the same port with the same cable.
• I have tried three different cables, all theoretically capable of Thunderbolt 4, DP, etc. I have tried plugging the Soundbar into all 3 USB ports on the monitor and directly in to the Mac.
• Audio out through the U3223QE's audio jack works perfectly.
• All other USB devices work as expected.
• Two other USB microphones (Blue Yeti and Obsbot webcam) work perfectly.
• USB audio-out through the Yeti works perfectly.
• When selecting the Soundbar microphone for audio in, the audio settings appear to stall  - if I select either the input tab or a different microphone nothing happens for around 10 seconds, then all works as usual. There is no onscreen indication that it has stalled.

This is the second SP522A that I have tried, replacing one that I believe was defective (it may have been - no lights came on at all). My best guess so far is that the Soundbar may be clashing with a different USB device or driver but I'm not sure how to diagnose that.

Help!