One way to do that is to export your PowerPoint presentation as Jpgs and then use a video-editing type of software like Premiere, FCP, Camtasia, etc. You lay down the music track and then plug in the individual slide pics in order at the time length you want. (I often have to do it that way). The modern versions of PPT let you export straight to video as you mentioned. Are you using a very old version of PPT? Also if you have a Mac available you can convert the slide and put them in Keynote and export to a QT movie.