How to measure 0.02V to 0.18V audio signal with arduino?
It might make more sense to use a precision rectifier and avoid the bridge. Below is an example that requires dual supplies (+/-5), for concept only, you can play with the values. It's a half-wave precision rectifier followed by a low-pass filter. You will have to consider the reference vs. grounding, many amplifiers are bridging type and do not have a common ground reference (from channel to channel or to earth). 

The rectifier amplifies the AC-coupled input signal by -R1/R3 = -47 for positive input. 







simulate this circuit &ndash; Schematic created using CircuitLab

Output with 100mV 1kHz sine wave input: