Why a balloon bursts with a pop sound when pricked with a needle ?

The air in the balloon is at a higher pressure compared to its surroundings . Elastic tension of balloon skin is pulling inwards uniformly .

When we stick a pin inside the balloon it creates a tiny hole . Now the rubber around the hole is not being pulled uniformly anymore . 

Hence the force imbalance increases . In a fraction of second , the entire skin of the balloon gets contracted to a point opposite to the hole .

The high pressure air inside the balloon is now free to expand . This creates a pressure wave that we hear as pop sound .

 Pop sound is created after a sudden pressure change due to the rapid expansion of the air inside the balloon . All this process happens in a fraction of second .