Spotify on BBB with USB DAC - no sound output

I’ve just put Volumio 1.5 onto a BBB rev C which plays out via an external USB DAC.
This works great for playing local files from the NAS, but although I have Spotify mostly working, it’s missing something somewhere…

So I’ve activated Spotify, I have my user and pass in there and I can view my playlists, so the login is definitely working. But when I play something from spotify, there’s no sound output. On the playback screen, I can see the names of the tracks and it seems to be playing through them ok in terms of progressing through the playlist and counting the time each song is played, but no sound output. I can skip, pause and play tracks and the web interface looks like it’s all working well, but it’s as though it’s playing via a different output device. I tried low res and high res modes, both the same.

Any ideas or any way I can get you some diagnostic info?