Though I do not recall this issue in PrE 4, later versions, prior to PrE 10, there were a few, Project-specific instances, where the two Fixed Audio Tracks had issues, when used. The "fix" for most, was to Move the Audio Clips FROM the Soundtrack, or Narration Track, to an unused "regular" Audio Track. In those cases, however, the issue was usually the Clips on the Soundtrack, or Narration Track, NOT playing - just the opposite of what you are experiencing.
I second Steve's suggestion to watch the Levels of ALL Tracks in the Audio Mixer.
Also, look carefully at any Keyframes (both Clip and Track) for Volume in the Tracks not playing. For a bit more details on Volume Keyframes (there are two - Track and Clip, that can have the same effect, but are different in several ways), see this ARTICLE.
Good luck, and please let us know a bit more,
Hunt
