Reduce background audio volume on select slides

There is no default method offered in the Captivate interface to control the Background Audio volume in the use case you describe.  But you can just insert an audio clip that has very low audio as slide audio on those question slides to achieve the same result. 

Just record something with your microphone without speaking.  Change the volume in Captivate to edit the clip if necessary.  Captivate won't know that you are not talking.  It doesn't measure the audio volume of the inserted clip to find out if it will be louder than the background audio.  As soon as it detects there is slide audio of any kind, it will reduce the volume of the background audio by the percentage you have set.