To enable HDMI audio output, open a song or video with audio then connect your HDMI cable to your computer and receiver. Click on the Volume () icon in the upper right, and click on Sound Settings. Click on the Output tab, then click on the HDMI / DisplayPort device for audio output.
You can also look for the Sound Settings by pushing the Ubuntu key and searching for Sound.
Click on the Output tab, then click on the HDMI device for audio output. You should hear the song of video playing through the HDMI device.