Op-amp for audio-arduino interfacing circuit
Probably there is some other issue. The LM392 op-amp should work for small input voltages (up to a volt or so peak) which isn't great but it's not zero. 

Check that there is actually AC voltage at C1, and that it is the proper value. The average voltage (unless you saturate) will always be 2.5V so you won't see it with just a multimeter.