If you want to experiment try:
Voicethread and Brainshark (use the free myBrainShark) - which are two free services that offer to add voice (and more) to Power Point. VT may not suit your project but it is a very interesting way to use PP's.
Another is Slide Effect, which exports to YouTube.
To make a seperate audio track - try both TrakAx or Audacity where you can add voice, loops and remix images and video free.