This usually happens when your computer is very busy capturing video frames and running other programs, so some sound buffers captured get lost. You can free some of computer time by capturing less frames per second and shutting down other, unnecessary tasks running.
Also, please read The frame rate you requested is too high error message article on this FAQ list on how to speed up frame capture operation on most computers - this will free a lot of processor time to also capture sound correctly.
If you already recorded a large file with sound out of sync, you may repair this e.g. with freeware VirtualDub tool, availalbe at
http://www.virtualdub.org/ . One of our users wrote:
Thank you! Virtual Dub worked extremely well for synchronizing video and sound in existing AVI file.
I just had to select:
Video->Frame Rate -> “Change so video and audio durations match”
Then
Video->Compression…->Microsoft Video 1-> Quality @80%, key frames every 100,
Then
File->Save As AVI
And that was it!