Time Duration in saved files

Kevin here with Prog Radio.  I had this same problem and it was driving me nuts.  In radio, you want very clean and tight edits.  I finally figured out what what causing the mystry space to be added to a track when saved.  It was the Reverb effect I had on the track.  When doing the mix down, there is the decay from the reverb that adds the "silence" at the end.  (It's not really silence, it's the decay from the reverb effect.). This can happen with any effect you have that carries over past the end of the track (reverb, echo, delay, etc.).

There are a few ways to fix this:

1. Remove or adjust any effect that could spill over when the track ends (Reverb, Delay, etc.).  The problem with this option, is you may still want the effect on the rest of the track.

2. Mix down the track, reload the exported audio file back into Audacity, cut the ending off that was added, and mixdown again, only this time without the reverb.  Since the reverb was added on the first mixdown, this keeps the reverb effect on the entire track other than the bit you snipped off at the end.  This option works, but is an extra few steps, so a bit of a hassle, especially if going lots of tracks.

3. Use automation on the effect.  This is the one I use.  In the multitrack editor, show the envilope for the effect, and simply drop it off a second or two before the track ends.  (See screenshot.). This will put the effect on the entire track, other than the last second or so, so you don't get the mystry silence spilling over past the end of the track.  

Hope that helps.