If you're on any OS 10.10.5 or previous with VSL software installed:
Open the AudioBox VSL control panel, and under the DAW return you will see pan settings in the channel strip. Set these so pan is in the middle at center.
If you're on Mac OS X 10.11 or higher with no VSL software installed:
This should not be an issue. If it is, then in Garage Band, set each channel to center pan and do a mixdown / bounce / save file. The output should be in mono.