Welcome to the forum.
Is your narration on the Narration Track? That is the obvious place for it, BUT some users have had intermittent issues with both the fixed Narration and Soundtrack Audio Tracks.
Usually, the "fix" is to Select all Clips on one of those two fixed Tracks, and then drag them to a "regular," free Audio Track, say Audio Track 2.
If you do have that narration on the Narration Track, try moving those Clips to a regular, free Audio Track. To save blank discs, instead of burning to a disc, at that point, Burn to Folder. When done, use a software DVD player, like the free MediaPlayer Classic HC, or one of the CyberLink DVD players (some free, some commercial), and test.
PS - When the problem develops with the Narration, or Soundtrack Track, it is usually Project-centric, and often the next Project will work perfectly with those two fixed Tracks. Why? Still a mystery.