Wondering if anyone has managed to make multichannel playback work using an HDMI connected receiver.
Using a Yamaha 1040 receiver connected via HDMI directly from MacMini using CoreAudio. Roon has the channel selection greyed out and set to 2.0 and downmixes all 5.1 albums (which are identified correctly as multichannel).
The same setup works fine using other software (e.g. Plex).
Try opening Audio Midi Setup, selecting the HDMI output and changing it to multichannel output from its default two channel.
Remove and re-add the device from Roon and you should find the channel selection no longer greyed out.