The crackling sound and the tiny sparks of light re-noticed due to electric discharge between sweater and body. The sweater is made up of wool and the shirt we wear is made up of cotton blended with some synthetic fibres. When we take out the sweater the friction is developed and the transfer of electrons take place from shirt to sweater, this results in the building of electric potential.

When enough potential has been accumulated while taking off the sweater it discharges and transfer of electrons take place. This transfer of electrons forms spark and let out sound energy and heat energy. Thus we hear the crackling sound and see the tiny sparks of light in the dark room