Hi
This is an inherent problem when synching a separately recorded sound track to a video. You could try changing the speed of the soundtrack keeping the pitch the same. This will lengthen or shorten the soundtrack, the minimum change being about 1%. The aim would be to adjust the total length of the soundtrack to make it the same as the video track length. Worth a try. You may not get it exact but it might be closer.( It's not clear whether you tried this with the sound or the video.)
The alternative is to cut the video at a few places where it won't be noticable, drag the right hand section up to the overlay track and pull it back sufficiently to re-establish the synch. Zooming in can help to get a small overlap if required. If the cut does jump noticably you could fade in the overlaid section to make it smoother.
Nat