Alternatively, it could be designed such that you would expect sound. Probably quiet at first, rather than SUDDENLY NOISE. Surprised me, Kickstarter has been a master of ambient video.
Super weird, I didn't get any sound in the slideshow at all, except when playing the Veronica Mars trailer and Werner Herzog video, which required user action to play. I went through the entire thing, not sure what I'm missing. What browser/OS are you using?
Latest Firefox on Windows 7. Happens on the launch satellites into space slide.