Finished the game, and I've found three hidden rooms. Maybe I missed some. Did the puzzles in each one, and when I completed it, it played a sound. It didn't play in one of the rooms though, but I'm pretty sure I completed it. Not sure what the significance of them are besides extra optional puzzles.

Anyone know what they're for, or if there are more? (I found the rooms with the coloured blocks grid picture, the tilty ball maze, and the tilty board with two balls)

I found those three (and like you, I could have sworn I'd solved the last one but no sound played. So I didn't bother hunting for more).

I think they're tied to Steam achievements, see "Behind the Scenes" etc at http://steamcommunity.com/stats/QUBE/achievements

Ah, that's pretty cool. The tilty ball maze looks like one of the beta puzzles. I looked at a couple of the Steam guides there and I don't see any mention of the other two secret rooms though. It is a mystery.

Looks like there's only the 3 of them (spoilers):

http://steamcommunity.com/sharedfiles/filedetails/?id=265092170

Same here.

When the first 2 were completed, a sound played, but the "2 ball/rotate"-puzzle made no sound when completed.