Before you start recording, making sure the armed track is a mono track, what options do you see in the tracks input drop-down? I assume there should be something like "1 /left/stereo" , "1/mono" "2/right/stereo" .
What's your pick? 
If nothing else helpsw, you can of course bounce the track to a mono track.

Add audio track and select input where you mic signal is come from(left or right) and you should get one single mono track:
 

also check your interfaces routing.
if it sounds right on the export but not during the mix it typically leads to a routing issue within the interface itself.

Sonar Home Studio XL, version 7.