Before recording voiceover narration directly in Captivate, you'll want to calibrate the microphone you intend to use from inside Captivate.
After selecting the slide to record the narration to, choose Audio > Record to > Slide.
Click the Calibrate Input button to open the Calibrate audio input dialog box. Ensure that your microphone is on, and not muted. Then click the Auto calibrate button.
Speak into the microphone. If you can't think of anything to say, read the text of the dialog box. The level indicator at the right may display various colors. When the text below the indicator turns green, you have calibrated your microphone.
After clicking OK, your chosen microphone is listed under the Device heading... and you are ready to record voiceover narration.