I created this as a new project in CC 2015, imported footage from three cameras, created the multicam sequences, then saved and closed the project.
When I opened the project again – two days later – to begin my edit, the audio in just one of the sequences had this left-channel defect. I corrected the problem using the Audio Clip Mixer and saved the project. After restarting the computer, the audio for all three cameras was once again all playing in the left-hand channel.
[I also captured separate audio from the house P/A system – imported, and synched it manually. This track (#7), as you can see in my screen capture, is displaying and playing properly.]
Don Wellman