Hi
Sorry. No idea at all.
However (and I know you say the preview plays OK..but...)have you checked the Options/Other tab to set the "Normalize Volume" box?
Tested a short clip which has an audio track to see if it behaves similarly?
Tested a short clip that you have muted and which has an added soundtrack on the lower soundtrack line?
Checked that you have not inadvertantly faded the soundtrack. (Right click it and select "Remove all fade points")
With some video formats there can be on occasions a short blank section of sound (1-2 seconds) at the end of each video clip requiring each clip to be lightly trimmed.
If none of these is the culprit....
What is the size and format of your imported video?
Is it the audio from Soundtrack 1,2,3 etc., or the audio from the Audio track which fades?
Is it only the END of the film that is affected?
What settings are you using when you save your output?
Nat