This largely depends on the soda. Most sodas are slightly on the acidic side. Coke, for example, has a pH level of 3.4. Freaky, right? Strong enough to slowly dissolve teeth and cause damage to bones...
Baking soda is a weak base, so it will react with an acid when the two are dissolved in a medium, usually water. Hydrochloric Acid, Acetic Acid, and Vinegar are all examples of acids.