The microphone is active when it appears to be pressed down, with a red dot. In exercises where speed is important, the microphone will activate automatically.
If the microphone is off, tap the mic button to activate it. The red dot should appear to indicate it is on.
For other exercises (like Reading and Revision), the microphone is OFF by default: the child has to tap the mic button themselves when they are ready to start reading. The mic button should show a red dot when it starts listening. That’s also what starts the exercise.