If I recall correctly (I don't have batteries in the 1998 version to confirm this), one of the buttons has different sounds depending on how long the button is pressed. If it is pressed briefly, it only has the walking sound; if the button is held down slightly longer, it gives you the walking sound as well as the "whining" sound right before the Snowspeeder is crushed underfoot.
It's akin to the 1998 FX X-Wing (which I just posted about in another thread) in that if you press R2's head down briefly, you get the "R2, fire up the converters" line before the engine whine, but if you hold R2's head down slightly longer, you get the engines to light up. I would assume, given the way it's being described, that the new AT-AT has a similar (if not the same) sound setup.