Here is a circuit that boosts audio levels to 0-4v.
http://macetech.com/vuinput01big.png
What I do not understand is why the battery is connected to the non-inverting input of the op-amp. I breifly tried this setup with my mp3 player and I nearly fried it

 

Where do you see such a connection? The non-inverting input is attached to the common point between R5 & C2.

 

There is that circular symbol labeled V2, is that a voltage source?

 

The circuit is wrong because it has nothing to limit the negative swing of the input signal. If the input was changed to be an inverting amplifier then a negative input swing would be allowed.
If the existing circuit used an additional negative power supply then it would work fine.

EDIT:
Audio booster? No.
It is a peak detector circuit with a DC output.

 

Must be why the output feeds an ADC.