Hi Michael,
There is no option to select the audio recording as stereo or mono at time of recording.
Can you tell me that have you installed captivate as a part of suite or it is a stand alone Product?
If you have installed Captivate as a part of E-learning suite there is a software called as Adobe Auditon which also gets installed as a part of suite , which can serve your Purpose , It can record as stereo as well as mono. and you can directly insert the audio generated from this software in Captivate.
And In Captivate also there is an option in Edit > Prefrences>Publish settings.
You can uncheck the Publish as Mono option while Publishing and check if it works for you.
Please check the screenshot
Thanks and Regards
Himanshu Satija