The MOTU Ultralight mk3 has 14 channels in 4 streams:
- Main Out (1-2)
- Analog Out (3-10)
- S/PDIF (11-12)
- Phones (13-14)
In the Audio MIDI Setup preference panel - I clicked on the checkbox to enable channels 1-10 (Main Out + Analog Out), then I went I selected the MOTU Ultralight from the Audio Interfaces list on the left, right click and select "Configure Speakers". From here, select "Multichannel > 7.1" and set each speaker option to a different output channel (front-center, front-left, front-right, etc...). Once each output is assigned it's own channel, then I can hear the file audio instead of the distortion sound! Yay!
So this is working, but my real problem now is how to mix all the channels back to stereo (I only have two speakers in reality) - this is probably specific to the audio interface itself and beyond the scope of this forum.
Good luck y'all, hope this helps somebody