What you're looking for is called a peak detector, or probably better in your case - an envelope detector. Pass this through an op-amp with adjustable gain to let you tune your sensitivity. R3 - your potentiometer will let you adjust the sensitivity.Your gain will be

.

Set Vdd to 5V and common ground with your Arduino.

It's unlikely your audio source is generating too much voltage given you're saying its not registering - but in the unlikely event you're above 5V put a reverse biased Zener diode in parallel with your capacitor to clamp voltage to 5V

^{simulate this circuit – Schematic created using CircuitLab}

What you're looking for is called a peak detector, or probably better in your case - an envelope detector. Pass this through an op-amp with adjustable gain to let you tune your sensitivity. R3 - your potentiometer will let you adjust the sensitivity.Your gain will be

.

Set Vdd to 5V and common ground with your Arduino.

It's unlikely your audio source is generating too much voltage given you're saying its not registering - but in the unlikely event you're above 5V put a reverse biased Zener diode in parallel with your capacitor to clamp voltage to 5V

^{simulate this circuit – Schematic created using CircuitLab}

Hit simulate to see what playing with values does!