I suggest you break your audio file into smaller clips. The next issue you were about to run into was going to hit once you started publishing your project and finding that the audio gradually became out of sync with the slides on some people's machines.
Captivate audio works a lot more predictably when you apply the audio slide by slide. If you ARE using distributed audio, then keep it to a handful of slides before starting the next clip. Another important trick is to make sure the audio clips have at least a slight (at least one-tenth second) gap between the start and end of the timeline they are on.