Try moving the soundtrack to a higher audio track (e.g. Audio 2). Both Narration and Soundtrack (below Audio 1) can go wrong.
Insanity is hereditary, you get it from your children
Also, go to the Window menu and select Audio Mixer.
Raise and lower the audio levels so that they peak at zero on the Audio Mixer but do not go into the yellow or red. Do not trust your computer speakers to give you a realistic impression of how your final DVD will sound.
And, if you've got any other questions, PLEASE don't write in all capital letters. Thank you.
Along with the other suggestions, and especially Neale's on moving the Soundtrack to a free Audio Track (some users have had issues with the fixed Soundtrack Track, I would convert that MP3 to a PCM/WAV @ 48KHz 16-bit, and replace the MP3. Most audio editors can convert the MP3's to PCM/WAV. Usually, it's just a simple Save to WAV, but some will require you to Export to WAV. This ARTICLE goes into more detail.
Thanks for you help
Sorry about the capitals
Moving music to audio one has done the trick
Thanks for your help
Every now and then, the Soundtrack and Narration Track (both Fixed Tracks, as they are automatically created, and cannot be Deleted) will act odd, and not work, as they should. These issues seem to be Project-specific, as they might work perfectly in the next Project. Sometimes, it seems that the issues are Asset-spcific, where another type of Asset, or maybe just a different Asset, even of the same type, will work fine.
When either fails to work properly, the "fix" is usually to just Move the Assets to a free Audio Track, other than either of those two.
As outlined in the linked article on MP3's, above, that file type can also be very problematic. I routinely convert ALL MP3's to PCM/WAV, and have never had an issue.