Buzzer is not working
First of all 

I'm going to talk about the power phase of your circuit. Look at the picture below 

This means that all the red lined tracks are short circuited (connected); thus all these npn transistors will work on their ON State which means; green, yellow, red, and the buzzer are going to be pulled up (ON State). So you have to add a switching device to determine on which level you have to power up each LED and when the buzzer will be pulled up also. And in your case you can use water level sensor.

Secondly

You have to determine the working mechanism of your circuit, which means when the level is going to rise from the green state to the yellow state, do you want the green LED to be still pulled up or you want to turn it off? this situation is going to determine if you have to add another components to your circuit or not.

Consider that you have decided to turn the green LED off (in our case), you have to make a truth table for cases you wish to light each LED On individually. We have three input levels and a buzzer. we will consider that the buzzer will be pulled up with the red LED. Also you have to know that when the level exceeds the green level and starts to enter in the yellow region you still have your green level sensor pulled up since water is rising from level to level. This understanding makes us familiar with cases we want the LEDs to be turned on and we also are able to draw our truth table.


You will find that the green LED will only turned ON when the green level sensor is ON and the yellow and red one is Off.

The yellow LED is ON when green and yellow sensor are ON and the red one is Off

The red LED and buzzer are both ON when green, yellow, and red are all ON.