Dear MitParmar
Hi
consider that the area is dark , the resistance of LDR , will increase and then the Transistor , will be short circuit (approx) .
Then 0.1 uf will have current ,(if BJt is off , the current of 0.1uf capacitor will be zero (because both side of it has same potential)). and then the 555 will work as a square wave generator at low frequency. ok you can hear its voice from speaker. consider the area is light. the LDR will be with low resistance. ok? and then the bjt will off and the current of 0.1 uf capacitor will be zero and then the 555 can not work as a oscillator.
Best wishes
Goldsmith