Alex, Curious how do use Kodi with you Mac Mini - as Kodi app running in OSX or using bootcamp and openElec installed on bare hardware?
If it's prior - I'd recommend trying the later
I have DTS-HD-MA passthrough working on my Asus Atom 2.1ghz /Nvidia GPU box and supposebly it works on Chromebox w/openelec.
I also going to assume you're using HDMI port for connection...