Creating "movies" in PowerPoint
Syncing audio with PowerPoint slides is very tough to do for one particular reason. As much as PowerPoint in its animation settings will tell you that the effect will take 0.5 seconds, it doesn't. Why? Not because of any problem with the software. But because the same effect will take a slightly different time on each computer depending on the speed of the processor, the amount of memory, the number and activity of other programs running at the same time, and a host of other variables that affect how fast the machine runs. Almost none of which you can control. And the 0.05 seconds off over 30 to 50 animations ends up throwing the sequencing off. Now, if you had just a 30 second segment, you probably wouldn't notice it. But over 7 minutes it grows to a noticeable issue.
So how can you solve this problem? Start thinking outside the slide. Think outside of PowerPoint. Use Windows Movie maker to assemble image files of the slides along with video you have shot that tells the story far better than movement on a slide could ever do. Assemble the images, add transitions between the slides, sync it to the audio, and output the final file as a WMV movie file. Then you can insert that movie on a PowerPoint slide and run it full screen as part of an overall presentation. To simulate builds on slides, create multiple slides, each with the prior information and one new build on it. Then, in the movie, don't have a transition between the slides so it looks like you are building points on the slide.
Remember, PowerPoint is built for presentations, not for creating movies. Use movie software and it will be much easier to create a synchronized end product.