The sound is out of sync

The sound is behind the video

Wireless headphones, particularly bluetooth, commonly add a fraction of a second of latency which sounds like the video being slightly out of sync. Correct it with Sync in the volume panel, adjusting it while the video plays.

An external audio file will not sync

Use Sync to automatically measure the delay. If it reports that it cannot be measured, try a different reference track: the video's original language usually works best.

If it still cannot be synced, align it with DescribeAlign.

An external audio track starts out fine and drifts

The two recordings run at slightly different speeds, usually one is for PAL format and the other for NTSC. Or they are from different edits of the same video.

Either find audio matching your copy of the video, or align it with DescribeAlign.