The following circuit diagrams are tone detector circuit diagrams which also known as sound activated switch circuit. Actually, these circuits use micro-controller for switching because the circuits were designed for robot start up activation. But you can build switching module using relay.
Tone Detector diagram 1
Tone Detector diagram 2
Tone Detector diagram 3
Tone Detector diagram 4
Tone Detector diagram 5
Source: Sound activated switch for robot
