I talked to @libreleah on the #minifree IRC and it turns out there isn't actually any HDMI audio through the miniDP++, according to the schematics there is no physical connection. So I can stop looking for a software solution.
But there's a way out! I'll find a dock for it. That will have working audio, I presume through a native HDMI connector.
Thank you Leah for the quick answer!