i have not used Sonar for video work however, i believe this may have something to do with the formats going into Sonar. The thing to note is that Sonar is not a native video editor.
first thing to note is the format going into Sonar:
1. use and external video encoder to export your video to either NTSC or PAL
2. during the export set the formats to the following:
a. NTSC - DV 720 x 480 (frame rate 29.97)
b. PAL - DV 720 x 576 (frame rate 25)
3. depending on the aspect ratios; set them accordingly to either letterbox or widescreen
4. ensure the audio output is the same as what you would set in Sonar for new projects (very impt)
a. if your Sonar is set at 48kHz 24 bit - do the same for the audio settings for the video export
5. save the particular video format as AVI - well because Sonar only reads AVI
6. when done, you should have a single AVI file in your Explorer Browser
7. open up Sonar - create a new project
8. before importing in anything goto Preferences to set the playback frame rate to sync with your video file.
9. all should be well
hope it helps...