If audio and video synchronization is always off by a consistent amount (constant lag), simply move the audio or video section slightly to align it better. If synchronization is off by a random amount depending on where you start playback, the file format may need to be changed.
Some variable bitrate (VBR) encoded files cannot be synchronized accurately and have to be decoded internally (buffered). If many VBR files or some large VBR files are added to a project, an excessive amount of computer memory will be used, adversely affecting system performance. Therefore variable bitrate files should be avoided.
You can use VideoMeld to convert a VBR file to a constant bitrate.
For audio files:
For video files:
Other software such as GoldWave may be used to batch convert many files, if required.