I'd suggest looking at the time allocated for each slide -
either right-click a slide and in the Properties, look at the time
allocated. You can type in a new number (in seconds).
I think a better solution would be to use the Timeline. When
you record audio into each slide, look at the waveform (probably
near the bottom row) and see how many seconds it is. You can
lengthen the time for the overall slide here to match the waveform
time requirements.
It sounds like you've recorded audio using Breeze
(Presenter), because in that application you playback the
presentation and click when you want to proceed. Captivate's a
little different. Once you get used to it, you actually have more
precise control