I just doublechecked - the flap sound can be heard inside the cabin when the flaps drive. It could be a bit louder, maybe.
1.) Position yourself in the cockpit
2.) Open the cockpit door
3.) Move until you are about mid-cabin (so you can hear the electrically driven hydraulic pumps through the cabin floor).
4.) Move the flaps (joystick button or default 1/2 keys)
Cheers, Jan