I had to do this for physics. I actually used windows movie maker, recorded my voice, then edited the pictures in so they went hand in hand with my voice and were related to what I was saying. I even had managed to put background musik in and added a movie clip. As for how to do it on Microsoft powerpoint I am clueless. On movie maker there's an actual button that says record voice and anda can record it there and make your power point on there as well. :)
