Sound Sensor

Hello,
I buy a Sound sensor for Arduino on dx.

But I have a problem.
I use analog Read and I see my sensor stay at 18-19 even if there is no sound or sound.
I don't understand.
Sorry I'm beginner and sorry for spelling mistake.
Thanks

Code:

int U;
const int sensor = 7;
void setup()
{
Serial.begin(9600);

}
void loop()
{
U = analogRead(sensor);

Serial.println(U);

delay(100);
}

What kind of arduino do you have? Pin 7, you sure it's analog?

I have a Arduino Mega 2560 ... I have already tested with another pin but no results :~