Hi Aranka
Here's how your project is going to be:
A sensor, maybe a condenser mic, will sense the sound and then pass its signal to a comparator like LM358. A threshold will be set ot this comparator which will decide when to trigger the 555 timer circuit. 555 will work in monostable mode and its output will be connected to a circuit which will operate the alarm.
The desired delay is calculated according to the formula: 1.1xRxC seconds.
Hope this helps 