recieving radio has its speaker jack connected to the arduino The circuit I have given is a peak-to-peak detector; it does not "bias to 2.5 V" but references to ground. If the audio signal is more than about 3.8 V peak to peak (2.5 V plus 1.2 V for the diode drops and a little more) it should register a HIGH on the input pin with a simple single digitalRead.
It does not include protection for the Arduino input but you would need to feed it 6.2 V or more to deliver more than 5 V to the Arduino pin. You could include a 10k resistor in series with the connection to the input pin if this was a concern.
Spk + and spk gnd
