yonargi1 wrote:
How do I insert a file into a lengthy video without making the audio go out of sync with the video?
Place the inserts on the overlay track. If you have but one video track, this will create track two.
Right-click the chevron to the right of the green arrow and "overlay on sequence at cursor."
Higher video tracks override lower tracks, without disturbing audio.
You can even fade the insert. Select it on the timeline, click its EFX button and click the TRANSPARENCY effect.
Open the preset options box and choose FADE IN/OUT.
Click the squiggly line to the right of the word "opacity" to see the keyframe trajectory.
Nat tells you how here.