20.04 sound over HDMI works like a charm.
I bet you can put 5V to the first pin as well, and have diminished volume audio over the audio pins.
I did and it works, and its
Mayfield audio with external speakers is the default, but if you don't supply 12V or 5V to the 12V power pin it will appear to not work.
But the sound over HDMI doesn't
work like a charm, as the setting in pavucontrol that changes it from from Mayfield to HDMI doesn't stick after a reboot.
The only reason I'm still screwing around with the DLI 20.04 image is I do have a use envisioned where I'll need the on-board sound, class-D amp and external speakers.
Standard Ubuntu 20.04 install has HDMI audio work, but you lose the Mayfield audio, dev kit camera module, GPIO, and Bosch sensor support. Not an issue for my first three Atomic Pi, but is for my 4th, hence I'm here trying to get the DLI LTS images working adequately.
Both the DLI and stock Ubuntu images have a serious issue with it locking up on shutdown or reboot. Not too serious if you are shutting down to pull the plug, but a showstopper for an IOT type application that might need to be rebooted by a watchdog timer. I've posted the solution I found in another thread here.
IMHO recommending 19.04, which is about to go
End of Life is terrible advice.
If you what to use USB AI accelerators on your Atomic Pi you'll need 18.04 or 20.04 for things like Intel OpenVINO or the Google Coral TPU accelerator. I'm finding the DLI 20.04 image is closer to usable out of the box than the 18.04 image is, especially if you need sound, but the version of OpenVINO for 20.04 drops support for the original Myriad NCS stick, it only supports the newer NCS2. Minor issue unless you already have used the NCS on a Raspberry Pi and are looking to
upgrade your base system.