Hey, I don't know if you've got this fixed or not (since this thread is a little bit old) but since it don't see the fix posted on here, I'll go ahead and share what's worked for me.
First off, this fix only works if you're on pulseaudio (the default sound server, if you don't know what I'm talking about, pulseaudio is probably what you're using). Go to the software manager, or your package manager of choise, and check to see if a package called pavucontrol is installed. If not, install it. Afterward, select PulseAudio Volume Control from the menu (if it's not there, you may need to right click the menu and select refresh). Inside of PulseAudio, select the configuration tab, and change the Profile to Digital Stereo (HDMI) Output. To switch back to your other speakers (assuming you have other speakers) you need to change that sound option back to whatever it was beforehand (probably Analog Stereo Duplex). You'll need to do this every time you switch as pulse doesn't detect when something's plugged or unplugged.