I get an error message with all the files I've tried so far (mp4, m4v, mkv)
Audio device is not configured.
You should configure your speaker layout with the "Audio Midi Setup" utility in /Applications/Utilities. Stereo mode is being used now.
(actually now gets cut off until you resize the window)
I am setup to use a stereo device already in Audio Midi setup (the first 2 channels of a 24 output digital device - MOTU 2408mk3) and the COnfigure Speakers is setup to use TDIF-A output (1-2), selected as stereo with left-1 and right-2. My audio and device work as expected after the error (playing fine in stereo).
If there's any info I can provide to help track this down let me know. I suspect it's likely due to the fact I'm using a 24 out unit and it's not seeing the typical "stereo speakers" type option from regular sound output.