I already amplified theses micros with opamp
You probably need to post a schematic drawing of your opamp circuit. Microphone audio is AC in nature and can't be read directly with an analog input pin as negative voltages are not allowed. There needs to be some kind of DC offset to keep the audio signal within the 0-+5vdc range of the A/D conversion.
Also you should state what you want the Arduino to do with the Audio signal it reads, as reading such a signal asynchronously is a rather random value not reflective of much more then that some random signal is present or not.