I just posted this in Facebook but seems relavant here as well. This has happened to me in at least three of the films I have scored in recent years.
Here is a heads up for anyone doing their editing using Avid. I have had one film this year and recently a version of a 2015 film play back in "mono" instead of "stereo". From my understanding from my filmmaker friend, Mark Baird who explained the issue to me, it seems that Avid defaults to having everything centered and so it will make mono out of a stereo soundtrack and you need to separate the channels either manually or with a pre-set. This will not only happen during the initial edit but also in subsequent edits. In the case of the 2015 film, the 168 festival version was opened and the front and end part of the film was removed but that also caused the stereo soundtrack to be reset back to mono.
So if you are an Avid user, keep this in mind especially if you are using sound other than dialog like foley, sound effects, and music scores.
And if you use anything other than Avid, check out your product to see if it does the same thing when it imports sound/WAV files.
It's a phase problem not an avid problem, look in the audio suite and use 'invert' that'll solve it. Could be the way your camera converts the audio or whatever software compression you're using. If not, check your wiring could be wired incorrectly internally it's happened before to me once.
Ken - not sure I understand your point. I write music and supply it normally to the audio editor as a single 16-bit WAV file (mostly 44.1 but sometimes 48). Most of the time, the audio editor mixes it in fine. Occasionally (already twice this year), the music was put into the center and sounded mono. My contact the last time this happened said it was a default in avid (or perhaps a preset), the imported everything to the center.
This has nothing to do with a camera - the audio is a produced stereo music file.
What daw are you using?