Oh man, thanks for bringing this nifty little tool back to mind! I'm not sure why I didn't explore it further (most likely the end of the school year), but it's still a great way to mix media and play with images and sound.
Just yesterday I was lamenting the fact that the GIF file format doesn't support sound, and today you've shown me a fantastic proof of concept of exactly what I was imagining! Is there a way to select only images that have the same size to improve the appearance of the animation?