Audio problem: The keys we see being depressed on the player piano never bear any resemblance to the actual notes we hear. (00:13:00)
Audio problem: For reasons unknown, the child's voice throughout this episode is played by an adult actress trying to sound childlike - and the unconvincing imitation is painfully obvious. (00:19:25)
Answer: It's explained in the episode. The aliens are experimenting to see how humans react to various scenarios. This just happens to involve them using super strength.
Thanks. It's been a long time since I saw the episode so I forgot.