As use of the iPad becomes prevalent in education, it is only natural that people will find ways to make it fully functional. At the moment, there is no way to have a PowerPoint presentation with audio while playing directly from the iPad. (The workaround is to import the presentation into Keynote).
However, there is a FREE online solution called "mybrainshark": http://www.brainshark.com/mybrainshark
The gist of it is that you upload your video to their website, use their tools to add audio, and then it creates a movie from your slide show. Viewers can view each slide individually or let the movie just run.
Then all you have to have is an internet connection to be able to play your PowerPoint presentation with audio on the iPad!