It has to do with the stereo, which I JUST found out myself (been puzzling over it for some time now >:|). Anyway, it was on the entire time, and then I turned it off, then on again, and then the stairs opened up.
I read, though, that someone said to skip to a certain song (didn't mention the station), or only turn it off. Just posting what worked for me. Hope this helps. :) If nothing else, at least you can experiment a bit with the stereo!