[ubuntu] Ubuntu 16.04 no audio through HDMI

I have Ubuntu 16.04 LTS installed on my laptop. The sound works fine through the laptop speakers, but when I plug an HDMI cable in and coect it to my Onkyo surround system, I get no sound.

I can bring up the sound app and select HDMI but when I click test I get nothing.

I've tried installing alsamixer and making sure that the volume is up and nothing is muted.

I know it's not the HDMI port because it works fine when I boot the laptop to Windows.

Anyone else run into this problem?