AudioServerException: Command '['aplay', '-q', '-t', 'wav', '-D', 'default:CARD=Headphones']' returned non-zero exit status 1

No audio output when running Rhasspy as an add-in in Home Assistant.
I’m running Hasio 2021.8.7 (in a docker container) on a Raspberry pi 4 (Rhasspy 2.5.10) - with phones plugged into the pi jack.
However, when I try the text to speech there is no audio output. If I select the JACK under ‘Audio Playing’ I get this error:
AudioServerException: Command ‘[‘aplay’, ‘-q’, ‘-t’, ‘wav’, ‘-D’, ‘default:CARD=Headphones’]’ returned non-zero exit status 1

I’ve played around with the different settings/voices/aplay etc but nothing seems to work.
Hovever, I have tried adding the OpenTTS Add-on - and that DOES give an output - so it looks like my hardware setup is OK
I have also tried installing Phasspy in a docker container following the instructions - and that does give an audio output.
Looking at that configeration, there a a large number of audio devices listed under Audio Playing/Available devices - BUT they don’t appear in the Home Assistant / addon installation.
Can anyone help me get the Home Assistant installation working please?