To make a flashing LED circuit, try googling 555 timer circuits. The 555 timer is a general purpose chip that will make sounds and flash LEDs. Now for the circuit: Firstly the buzzer is a dc buzzer, so it requires a constant voltage across it to sound. if the N/C connection is short circuit, then the base of Q1 is at 0V, so its collector is at 0V therefore the base of Q3 is at 0V which means Q3 is switched off, so no current flows through the buzzer. When N/O is open the base of Q2 (which is a pnp) has no pull down, so Q2 is off, so Q3 has no drive to it, so the buzzer is off. removing N/C pulls the base of Q1 (and its emitter) high thus switching on the base of Q3. Likewise, shorting N/O to ground switches on Q2, driving volts to the base of Q3, thus switching it on
Hope this helps
Simon