Is turning off engines before landing "Normal"?
There's virtually no chance of the pilot deliberately switching off all 4 engines, and even less chance of all 4 engines failing at the same time. It's not entirely impossible for it to happen, but it definitely wouldn't be quietly ignored within the aviation community. I promise you that you would've heard about it afterwards.

That said, the A380's engines are modern, quiet, and actually mounted quite a long way out on the wings, this makes them fairly quiet when idling, even when compared to other aircraft. This is particularly noticeable after a long flight where they've been running at fairly high power for 8 or more hours, and is even more the case if you're not used to the A380, eg if you usually fly on common twins such as the A320, 737, 777 which are much louder within the cabin.

I suspect, but can't be sure, that the engines were simply pulled back to idle (minimum power/thrust) and it sounded much quieter than you expected.

But no, the engines were almost certainly not simply turned off, that just doesn't happen. The only thing I can think of that would cause that would be a flame-out, and you'd definitely notice that!