I did a quick thing yesterday for upload to YouTube. I mixed and edited audio and video from two different cameras. Both were downloaded from YT.

Since the project was to be a quick and simple thing, I didn't do my usual setup with the audio tracks. I took the two audio files into Audacity and mixed them,cleaned them up a little, etc. I took the finished tracks into lightworks and synched them with the video edit. I inserted the audio like you do with video over black. When I went to play the edit from the beginning, the audio (starting some seconds in) stumbled to start. I was still synched once it got going.

As it turns out, I needed to add a few seconds of silence in the empty space (using replace) at the beginning before the "inserted" audio. It threw my synch off again, but it started properly.

I didn't try to export before I fixed it so I don't know how it would have ended up. It looks like empty space in the time line is not the same concept as black video. It acts like it takes a while for audio reading to catch up when empty space is involved. At the beginning in this case.