You are very infrequently sampling the microphone, so of course the results seem random.
int sensorValue = analogRead(A0);
// print out the value you read:
Serial.println(sensorValue);
delay(1); // delay in between reads for stability
Just printing out "sensorValue" at 9600 Baud slows the program down enormously, and of course, the idiotic delay() doesn't do you any good.