To enable HDMI audio output, click on the settings icons in the upper right, and then click the gear icon().
You can also look for the Sound Settings by pushing the Super key or and searching for Sound.
Click on the Output tab, then click on the HDMI device for audio output.
The program PulseAudio Volume Control is super helpful in figuring out which program is producing audio, where that audio is being routed, defaults for audio outputs, and volume levels for every source, input and output. It can be installed using the software center, or with this command:
sudo apt install pavucontrol