It sounds like your audio channels were recorded out of phase.
So when you mono them they cancel eachother out, hence the low level.
When you pan them left/right for stereo the cancelling effect no longer happens so the level appears more normal.
Depending on what you have recorded on what track you may be better off just using one or other as otherwise you may hit upon this issue later.
Or you can try phase reversing one of the tracks if you need to keep both.
best wishes
Neil
http://www.patience.tv
