Here is Leigh's advice on how to attach vocals to a PowerPoint presentation.
One way is with the slidecast feature in slideshare: http://www.slideshare.net/slidecasts
The first step is to record audio for your slides. My advice is NOT to use the inbuilt audio record in the modern version of PowerPoint. Use an audio recorder and create a seperate audio file to add later. A free and open source audio recorder = Audacity