I never heard of this but it seems to be the case on my system.
X3 needs the audio track that is imported with the video track to determine how fast the video should play back.
Also noteworthy is that the audio from the video render is blank ... nothing on it ... just a 44.1 / 16 audio track.
No ... 48 makes no difference.

Trouble shoot process:
Open blank project
Set Clock to 30fps
Import a video with the audio (.mp4 at 30 fps)
Video plays back correctly

Import a video with the audio (.mp4 at 30 fps) but then delete the audio track
Video laggs a lot ... almost slo motion ... frame counter in video window doesn't match the frame being seen.
 
Import same video and not the audio
Video laggs a lot ... almost slo motion ... frame counter in video window doesn't match the frame being seen.

Anyone else?

There should be a freakin' huge red flag somewhere that says:
"Your imported video NEEDS it's original audio track to play back correctly in Sonar X3."




 
 

Bump!

Has no one encountered this?

There is no correlation between the audio track from the video and video playback. 
Video is slaved to the audio clock from SONAR. Check if the sync mode is "audio" in the sync module in the control bar. Does it work if you add an empty audio track to the project?
 

Yes. Sync is set to audio.
Re-testing from scratch again.
blank project
import video - .mp4
verify sync is set to audio = yes

test playback before deleting the audio imported with the video = playback is correct.

delete audio track = playback not good
lagging and out of time
Sync is still set to audio

add blank audio track ... no change
still lagging
attempt to record empty band of audio on new track... no good ... still lagging

Not really a big deal now that I know to never delete the audio track that's imported with the video or never to choose to not import audio with video.
It just had me going for a while trying to figure out why the vid. was playing out of time.

Did you try it to verify that it works without the original audio?

P.S.
If I undo all the way back to right before deleting that audio track it then plays correctly.

Noel Borthwick [Cakewalk]
There is no correlation between the audio track from the video and video playback.

Theoretically is assume ... but have you (or anyone else) tried it to verify that it works correctly on your system?