ThinOS 9 - Soundbars not showing in citrix session

Need some help getting soundbars to work with thin clients - 8.6 & 9.1

I've got a new SB521a soundbar for testing, to use with the new P2422h monitors. (We were using AC511 soundbars with the P2419H monitors, which have been discontinued)

The new thin clients are all on ThinOS 9.1. The existing TCs are all 8.6

8.6:

Old soundbars work fine

New soundbars:
Don't show up in the win / citrix session audio settings
Don't output any audio (defaults to internal speaker)
Don't show in the device audio settings

9.1:

Old soundbars:
Output sound, but can't control volume with soundbar volume knob
Doesn't show up in citrix/win audio settings (Only ever shows HDX Audio)
Does show up in device audio settings

New soundbar:
Outputs sound but only very quietly (compared to test on laptop)
Doesn't show in citrix/win audio settings (Only ever shows HDX Audio)
Does show up in device audio settings

 

I've tried putting: "DELL Slim Soundbar SB521a-5;DELL AC511 SoundBar" in the Playback device field in the WMS (Peripheral, Audio) (Just guessing, in the absence of any actual instructions anywhere about how any of anything in the WMS works)

I've also tried USB redirection excluding audio and not excluding audio.