My mistake about the sound card - I was forgetting that audio.device was a Paula thing and not AHI. Paula is definitely] supported in AmigaOS 4, though.
I'm pretty sure audio.device redirected to AHI in OS4.0, although software that opened audio.device and then hit Paula directly wouldn't redirect without NallePUH.
No idea which AHI unit though - probably Music, although could have been Unit 0. I'd check all AHI units are set correctly before doing anything else.