Surround sound not working via HDMI

I have a Raspberry Pi 3 hooked up to a multi-channel amp via HDMI. I cannot find where/how to configure Volumio to play multichannel flac files - it plays everything in stereo.

The hardware definitely works - l just upgraded from OSMC to Volumio because of network stability issues (frequent resets). OSMC has an advanced set-up mode with an audio screen that allowed me to specify the surround configuration (4.1, 5.1, 7.1 etc.) and when set, correctly played 5.1 flac files.

Could somebody please point me to the equivalent screen in Volumio. Thanks

Volumio only supports 2-channel (stereo) output, your hardware may support more but that is irrelevant.