Solved: surround sound using HDMI with Apple USB-C Digital AV Multiport Adapter

I wasn't able to output to more than 2 channels using the USB-C Digital AV Multiport Adapter (Model A1621).

I tried every possibility (Audio Midi setup, resetting, different cables), but wasn't able to output to any other channel.

I read on https://support.apple.com/en-us/HT207806 that there was a newer adapter, so I tried that one (Model A2119), and surround sound (8 channels) worked out of the box.

I hope this information can save some of your time. And hopefully Apple will update it's article, as "Model A1621 supports HDMI 1.4b" is not correct.