When you revisit a slide in a presentation, PowerPoint only shows the slide as it was finally revealed at the end of any animations, sounds, or movies on the slide.
There's no direct way to re-start the animations and view the slide from the start, but if you insert a blank slide ahead of the slide with the animations, set it to automatically transition to the following slide after 0 seconds delay, you can jump back to this slide, it will automatically play forward to the following slide (the one with animations) and you're in business.
Did this solve your problem? If so, please consider supporting the PPT FAQ with a small PayPal donation.