i tried to simulate the circuit below with the Proteus 8 Professional , but i didn't find the SL100 transistor so i replaced it with BC547 , is it ok? and when i give the pin 4 high , the LED starts to pulsing but the buzzer doesn't work .
The transistor is being used as a switch so you should be able to use almost anything provided it can handle the current - not that a 1/2W speaker needs much current. As for the lack of sound, are you using a speaker or a buzzer, if your using a speaker, the frequency driving it may be too low, if you can see the led pulsing, it's got to be below 50Hz, not many small speakers work well at those frequencies. Try fiddling with VR1 to see if you can the circuit to buzz at an audible frequency.