Upon dying, what occurs to the soul?

According to the Hindu tradition, a soul (from the stage of a protozoan, perhaps) attains competence for a human body after undergoing 8,400,000 births (in lower forms of life).
Brihad Vishnu Purana gives the details.

http://www.vedanta-newyork.org/articles/concept_of_evolution.htm

http://www.harekrishna.com/col/books/KR/cb/chapter1.html

http://www.stephen-knapp.com/vedic_culture_hinduism_a_short_introduction.htm

Upon dying, the Soul leaves the body in a flash. All 30 trillion cells have the Soul energy, but at death, the Soul departs, there is no breath. The Soul merges with the Soul that is present everywhere. Unfortunately, because we cannot see it, we don't understand it easily. But it's easy to understand with the example of air in a balloon. What happens to the air when the balloon bursts? Simple. It merges with the air that is everywhere, so does the Soul.