Though a complex project may play on the timeline, the export may differ. To help prevent that, follow the suggestions in these threads.
Adding background to previously exported video clips... https://nch.invisionzone.com/topic/34261-export-overlaid-video-add-background-later/?tab=comments#comment-92792
6x6 https://nch.invisionzone.com/topic/32584-virtual-choirenhancement/?tab=comments#comment-90215
http://nch.invisionzone.com/topic/31015-virtual-choirjocelyn/?tab=comments#comment-81660
http://nch.invisionzone.com/topic/31417-no-video-in-exported-file-choir/?tab=comments#comment-83013
http://nch.invisionzone.com/topic/30975-virtual-choirdiana/
http://nch.invisionzone.com/topic/31800-no-audio-waveform-when-sequence-is-a-clip-choir/?tab=comments#comment-84399
http://nch.invisionzone.com/topic/31823-100-clip-multi-screen-method/?tab=comments#comment-84501
http://nch.invisionzone.com/topic/31800-no-audio-waveform-when-sequence-is-a-clip-choir/?tab=comments#comment-84502
Synchronizing tracks...
One way to approach this would be to import two tracks. Then set one track where "row" begins at the head of the timeline. PLAY. Drag track two until the audio matches. It may be necessary to clip off 'garbage' at the head of track two so it can be dragged far enough to the left to match up. Takes some experimentation.
Lining up audio waveform peaks - one above the other - makes this easier. In some VP versions, right-click on the audio track and SYNC AUDIO.
An alternative would be to load all tracks, then solo tracks one two. Match them up, then solo tracks one and three, and so on.
After each is in sync you might right-click to the left of the track and LOCK TRACK. This will lock both audio and video for that clip.