Extending diode with a buzzer
I traced out part of the circuit and it appears that the LEDs are turned on by pulling the 74HC595 outputs to GND, not VCC. If this is correct then your 'Q1' transistor will be turned on whenever DL12 is not turned on. Furthermore, 74HC595 pin 6 is pulled low to supply power to the LEDs, so your 'Q2' transistor will only turn on when (if ever) power to all the LEDs is turned off.