exporting results in both mono and stereo mp3

I’m exporting a series of voice recordings, done in mono, to MP3 for ACX. For some reason some of the files are converted to stereo, while others are mono.

How can I be sure all files are converting to the same format?

I’ve tried both export selection, and export multiple. I’m using the constant bit and joint stereo settings. The tracks are mono and there pan is at zero. Unfortunately I find out that some are coming out as stereo from the ACX QC team.
Each track has been normalized and compressed using the same settings for each.

Im using Lion 10.8.5
Audacity 2.05

If all of the audio tracks in Audacity are mono, AND the Pan slider (http://manual.audacityteam.org/o/man/audio_tracks.html#pan) is set in the central position, then the export will be mono.

You can check the file format after exporting by using a program such as MediaInfo http://mediaarea.net/en/MediaInfo (The Mac download link will redirect you to the Mac App store - there is a small fee)