Q: HDMI audio not working (showing up as displayport)

Hi, I have a 2015 Retina Macbook 12". I've connected an LG IPS 4k monitor via HDMI (using's Apple's USB-C multiport adapter). The video looks incredible but I can't get any sound.

The volume control on the menu bar is greyed out. In the Audio settings, the LG monitor is visible as an output option, but as a Displayport connection, not as HDMI. That struck me as odd and potentially indicative of a problem.

Would appreciate any help.