Richard,
Thanks for the post. So, after many attempts, I did get this working, but everything has to be set up quite specifically.
Adobe PP 2015.4 (10.4)
OS X 10.11.4
Avid Mbox Driver: MboxPro_OSX_1.3.5
Earlier versions of the driver don't load the MBox Pro preference pane into Systems Preferences.
IN the PP Pro Preference Hardware set up, set Master Clock to Built in Line Output. IN the Mbox Pro Control Panel/Application, find the Set Up tab and set Clock Source Clock to Internal. If you get these wrong, video and audio will not clock properly and will play back jerky with no audio.
Create a Multi-Channel Audio timeline with 8 channels in PP and populate that with your discrete tracks.
Map those PP channels to the specific output channels on the Mbox in PP Audio Hardware, which should show all 8 channels. For some odd reason, SPDIF shows up in the middle of the channel breakout. Once this is correct, you should see levels playing both in PP and in the Software Audio Mixer of the MBox App. You may also need to pick Pro Tools 8/8 device in the Mac Audio Midi Set up Application.
On a separate note, I'm missing the Avid Mbox 3 Pro Breakout cable. If anyone knows how to source one, please respond here!
Thanks.