"the thing is that I clearly mentioned in the post, "what should I do after the "MICROPHONE". No body told me yet."

In that case, I'm sorry - I misunderstood your Post.

You said,
"Does any body know any type of sensors which measure noise level? ... My first thought is to use a microphone, some amplification, A/D conversion, averaging and then calibrate it with a standard Noise level meter."

That sounded to me like you pretty well had the idea of what to do after the sensor; it sounded like just a question about the sensor.
But I did add one think you need to do after the microphone - that was the issue of weighting

Whether you decide to do the weighting by analogue filtering (before the A/D) or digitally (probably software) is up to you.