As I understand it, it has more to do with your own audio
capabilities on your PC than anything else. I got this from
TechSmith when the option wasn't available for me using Camtasia.
As it still isn't an option for me in Captivate, I can only assume
their explanation was correct.
There are other options, such as acquiring a patch cable that
runs about $5 from Radio Shack and running from the Speaker out
jack right back into the Mic in jack. Or, you could purchase a
mixer that would do this as well as provide an ability to blend a
microphone input in there too.