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 

<img src="https://latex.codecogs.com/gif.latex?V_{out}=&space;V_{in}(1&plus;\frac{R_2}{R_3})" title="V_{out}= V_{in}(1+\frac{R_2}{R_3})" />. 

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 

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![schematic](https://i.sstatic.net/Fpsla.png)

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Hit simulate to see what playing with values does!

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/L00Fm.png