This circuit detects the presence of audio (voice) on the output of a scanner. If the scanner stops on a "dead carrier" or noise, the circuit mutes the speaker to avoid annoying noise. U1 amplifies speech and drives rectifier D1/D2 and switch Q1. Comparator U3 drives speaker switch Q1 and indicator LED1. Q2 completes the speaker path to ground. U2 is an audio amplifier to drive the speaker. R3 is a volume control. PL1 connects to the scanner speaker or to the headphone jack.