The sound simply doesn't work, I'm using HDMI on a large flat-screen TV. Earlier today I had it working with HDMI on a smaller TV that is not a flat-screen. I have heard that it may have something to do with the resolution.
Open raspi-config by typing
sudo raspi-config into the terminal.
Select option number 7
Advanced Options by moving the red bar with the arrow keys, and press
Select option A4
Audio, and press
Select option 2, and press
Enter once more. It should now output audio to HDMI.