Depending on the size, and the number of people involved, recording a simulation in captivate with narration can be a little frustrating.
I've recently started using a new method which is working out really well. Since i'm not a huge fan of Captivate's audio editing tool, I try and record all the narration separately in something like Audacity (free) or Audition (not so free).
With some nice narration recorded, I then start recording a new simulation while listening to my recorded track. The trick here is to select "record narration" before you start recording, this way the slide timing will match the audio you are listening to.
Once you've created your simulation, all that is left to do is to go into Advanced Audio and delete all the audio tracks that were recorded (which would be blank or just a poor recording of what you were listening to). Then import your recorded narration to slide one and select retain current slide timing.
You may need some slight tweaks depending on how good your video timing was but most things should line up great for you. Hope this helps, and feel free to share any other practices you use that work!