this is found in chapter four. Make it up to the pillar puzzles and at the top of the steps next to the power switch is the microphone.
Interact with it once and she will talk into it. Next interact with the power switch and then interact with the microphone. This time she will sing into it.
Warning: do this before turning on the main power (Completing the puzzle).W Andersson
says that he had to interact with the microphone several times for her to sing. My solution is based on what happened with myself.
Further edit: spam away if it doesn't work.