Have a look at the audio mixer pan controls, perhaps some are panned one way and some the other.
Another thing to check is to make sure they have imported as mono clips on to mono tracks and not as mono clips on to half a stereo track.
The track mapping you're seeing is curious, and I don't have an answer for that, but audio pan info is set in the Audio Mixer, and by track, not by clip. Start by looking there, and set your mono track panners to 0.0.
You can also automate panning, and/or set manual keyframes for pan in each mono track, by setting your Timeline track setting to Show Track Keyframes, and then select Panner Pan from the gray menu that will appear to the right of the track panel.
I have a number of mono sound clips
Terminology is important here. Take note that a stereo clip with sound only on one channel is still a stereo clip. A genuinely mono clip will only go onto a mono track, and will come out both speakers.
Just to follow up - yes, these were mono clips on mono tracks.
Richard was absolutely right: I should have checked the audio mixer and pan settings, but didn't bother since I generally don't even have the mixer open or the pan settings displayed - I'd normally hand over to a sound designer using ProTools for finishing and mix. So there's no way I'd even have modified the settings, right?
Wrong. No idea why, but when I opened up the track pan display I found dozens of panning keyframes entered for those seven tracks, stretching well beyond the end of the current sequence duration. My best guess is that these were somehow left over from the FCP XML import - I hadn't done any stereo mixing in FCP either, but maybe this is how Premiere interprets certain kinds of FCP audio information?. That could be way off, though.
Thanks for your replies.