Change default audio device to HDMI in Mythbuntu

I'm running Mythbuntu 16.04

The output of
wget -O alsa-info.sh http://www.alsa-project.org/alsa-info.sh && chmod +x ./alsa-info.sh && ./alsa-info.sh --upload
is at:
http://www.alsa-project.org/db/?f=c6e961b1c4eef480b1ded390f80fba160b776131

My HDMI audio output is working - I have changed MythTV to use:
ALSA:dmix:CARD=HDMI,DEV=3
and sound works correctly in that program.

$ speaker-test -c wav -D plughw:1,3
plays white noise as expected.

But I don't know how to set this as the default device so that things like YouTube etc in web browsers work.

I tried creating /etc/asound.conf with the following contents:
defaults.pcm.card 1
defaults.pcm.device 3
defaults.ctl.card 1

and that didn't work.

Any help gratefully received!