You could do this if you export the presentation as a Quicktime movie, then add a sound track in video editing software. Alternatively, one could capture the play through in an application such as ScreenFlow, then add the desired sound track to segments of a slide.
It is also may be possible to put the contents of the 20 slides on one slide using builds.
There doesn't seem to be a straight forward way to do it, but it is possible, though it might not meet your needs.