As I usually provide support on public presentations at some conferences and seminars, I find that most presenters' voice narration in the speech would be recorded live for further references. Actually I recommend that the presenters should record narration by themselves with the microphone and PowerPoint's internal recorder. In this way, your speech will be perfectly matched with every slides of the presentation, and the narrated presentation could be better materials for more absent audiences. Read Virginia Tech's comprenhesive instructions for how to record narration during presentation.
If the voice narration has been recorded independently, or some content needs modification, you can also manage and split audio into segments related to each PowerPoint slide. Audacity (http://audacity.sourceforge.net) is the recommended audio editor for everyone. Please notice that all sounds need to be embedded in presentation for integration, refer to Office PowerPoint Help "Embedded and linked sound files in a presentation" at http://office.microsoft.com/en-us/powerpoint/HA012303071033.aspx
Someone may ask me, why not record live speech with presentation as video directly? Video can record everything in the conference, but your final presentation don't need all the things definitely. In most business occasions, visual slides and voice narration are the most important parts in the presentation, so narrated presentation is enough, besides video files usually face the encoding problems. How do you think about this?
Introduce an experiment presentation by Bill Conerly at Wichita State University's annual Economic Outlook Conference:
William Peterson Presentation Veteran