Some users have encountered issues with good Audio Clips placed onto one of the two "Fixed Audio Tracks," Soundtrack and/or Narration.
These issues seem to be very Project-centric. The Fixed Tracks will function fine for a time, and then develop problems, or the user will immediately have problems with those Tracks in one Project, but not in others.
When issues develop, the workflow, that has always fixed things is to locate an empty, "regular" Audio Track, such as Audio 2, or Audio 3, and then Move the Clips from the Fixed Track to that. If one does not have enough empty, regular Audio Tracks, they can be created, with New Track (location varies by version).
This problem has been around for several versions, and is so hard to track down, because it will effect one Project, but not the rest. To date, we have not been able to track the problem down, so do not have a real "fix," but only the workaround.
Though many have never had the problem, because of the number of reports, they just do not use the two Fixed Audio Tracks, Soundtrack, or Narration.