According to the New York Times, expired soda is safe to drink. An article about expired Coca Cola stated that even soda old enough to grow mold poses very little health risk.
Most soda companies do not actually label their sodas with expiration dates. Rather, they label soda boxes and cans with best by dates. There is an important distinction between best by dates and expiration dates, as an expiration date warns consumers of potential health hazards, and a best by date is meant to inform consumers of potential taste degradation as a result of aging. As sodas usually only have a best by date, this means they are safe to drink well past the labeled date.