Thanks everyone, I've been digging into the file I was sent for more info and googling like a madman, I might be on to something ... but I badly need some sleep.
I'll report back here some time in the next couple of days or so once I get a chance to try some things and talk to the people who produced the file.
Here is what I've found so far from digging around, and a couple of emails back and forth.
It may not help me with this particular project, but may help with this client and future work from them - we'll see :-)
The mp4 file was produced using OBS (Open Broadcaster), and the audio streams are all AAC format, which is where the issue probably is for Sonar. I believe there is a reasonable possibility that if the audio streams/tracks were PCM rather than AAC encoded, Sonar might just suck them straight in without any problems.
They can't provide me a file with PCM audio tracks because it was from a live session and this is what they recorded.
It looks like OBS can indeed output multiple audio tracks to standard uncompressed PCM, but not with the settings they are using (and they don't seem to have a handle on this at all).
I'll install OBS in the next day or two, see if I can produce a video file with multiple PCM audio tracks. If I can do that, I'll check what happens in Sonar.
I'll report back as soon as I get through all this :-)
Cheers - Cliff