Sound over HDMI on Nexus 7

I can't get any sound output over HDMI on my Nexus 7 (2013).

First, I tried with Ubuntu Touch booting using Multirom. The display output was fine, but no sound.

Then, I did a proper installation using ubuntu-device-flash. Again, the display output was fine, but no sound.

(In both instances, the sound comes out of the tablet speakers.)

In Android, the sound comes through just fine. So I don't think it's an issue with the cable or the slimport adapter.

Any ideas?