Ok, so I dropped a stereo track into Audacity, panned it all hard left.
Once I mixed down the Audacity track to WAV and dropped it into Sonar (drag-and-drop directly from my desktop) it sounded perfect: Right was mute and Left was as I expected with no bleed.
However, if I dropped the file into a track from Sonar's Browser, the left side of the track was about 60% of it's original value. The right side still remained mute.
This is getting weirder and weirder...