Circuit given below is configured on astable mode of operation. In place of a fixed resistance, LDR has been kept for getting variable resistance for the 555 timer to work in astable mode. Now, as the resistance increases or decreases in accordance to the light falling on LDR, we hear sound of different frequencies.
The logic of sound generator using 555 can be used to make a simple toy organ. As we press different keys (Push to ON switch), the resistance between pin 6 and pin 7 varies and that produces sound of different frequencies.
Here, you can see that the logic of a blinker has been used to make a sound generator. Use of LDR makes us easier to understand that the sound frequency varies according to the resistance variation in LDR.
The sound generator is based on astable mode of operation.
I have built a DIY kit using this schematic. You can read more about the project on this page: https://www.buildcircuit.com/diy-kit-65-ne555-based-sound-generator-diy-kit-with-a-photoresistor/
We can make a simple toy organ using the logic of sound generator. As we press different switches, the output frequency of the circuit varies and it produces sounds of different frequencies.