Glad to see additional work being done on Ubuntu MATE for the Pi. I tried to edit the config.txt file on the ROOT partition to enable HDMI audio, and now I cannot get past the rainbow screen. I also tried deleting the edits from my laptop and still cannot get past the rainbow screen. I tried undoing edits to a config.txt file under the ROOT partition called /boot/firmware/config.txt, but strangely when mounting it on Linux, even while browsing as root, it's nowhere to be found. I will reformat. Does anyone have precise instructions on getting audio working? I will backup this time around. Luckily much wasn't impacted because as of now, I primarily use Raspbian.
Another question - when official 18.04 drops for the Pi 2 and 3, will it include built in HDMI audio support and built in bluetooth support?