Yes, that happens -- sometimes, the rehearsal timings may be different even if you use the same version of PowerPoint. There's not much you can do -- but do follow these guidelines:
- Keep a gap of two to three seconds after each slide to compensate for change in timings.
- For slides that are full of custom animations, keep a gap of 5 seconds before the next slide plays.
- Test the rehearsed presentations on several versions of PowerPoint to make sure that the timings work well.
- Test the rehearsed presentation on both Windows and Mac versions of PowerPoint if you work in a cross platform environment.
- Windows Movie Maker
- Photo Story
You may also use the best of both worlds -- create a presentation in PowerPoint, and then create a movie from that presentation with added music in Windows Movie Maker. You can find a tutorial on creating such a Windows Movie Maker movie file from existing PowerPoint content here...
Mac users can use iMovie (included with most Macs) to achieve the same purpose although PowerPoint:mac allows direct movie export.