Another great, super easy way to insert narration in your PowerPoint is to upload it to Knovio. The interface is great! You can narrate each slide and easily re-record as needed. Then you can export your slideshow as a video.
NOTE: PowerPoint does not recognize some audio file types, such as .m4a. If you have an audio file from "voice memos" on your iPhone, you'll need to convert that file to an mp3 to use in PowerPoint. Zamzar is good web application for converting audio files.