In a world where mirrors do not reflect visible light but instead reflect sounds, Keiji stands in front of a large flat mirror and claps his hands once. Lian, standing behind Keiji but still in front of the mirror at an equal distance, hears the sound of Keiji's clap. What does Lian hear and from where does the sound appear to come?

Respond very briefly.
Lian hears two claps: one directly from Keiji and the second from the mirror. The sound from the mirror appears to come from behind the mirror's surface, as if someone identical to Keiji is standing the same distance behind the mirror as Keiji is in front of it, and also clapped their hands once. The sound waves from Keiji's clap travel forward, hitting the mirror and are then reflected back, thus reaching Lian as an echo.