I want turn the led off only when the input is low for a prolonged period of time and not when there are ups and downs in sound signal My project is baby cry detection,when the baby cries the led must turn on and when it stops it should turn off, but I want it to turn off only when the baby has stopped crying , not when there are ups and downs in the sound . When there is a high in baby sound the led turns on and when there is low the led turns off.  I want the led to stay on and turn it off only when the baby has stopped crying completely . If the input from sound sensor stays low for a prolonged period of time then the led should turn off , how should I write a code for this condition ?
It's very related to the debounce algorithm (used for debouncing switches/keys), but consider a button 'bounce' as a cry. The time is much longer (seconds, to even minutes).

See here the debounce explanation:

https://www.arduino.cc/en/tutorial/debounce


As soon as the baby starts crying, you want the LED to be turned on (almost) immediately, so use a short 'debounce' (or in this case: cry on) time for a rising edge (start crying), something in ms or a second maybe (I assume it is that time critical).
However, for detecting stopping crying, you probably want a much longer time, maybe even a minute (but I leave the exact timing requirement up to you).