Getting buried alive probably hurts. You would involuntarily breathe in dirt particles in an attempt to take in as much oxygen as you can, but the dirt would probably get trapped bit by bit inside your lungs.

If they really wanted to torture the guy, they would do it very slowly, not all at once. Centimeter by centimeter around the guy. Little by little as the dirt rises up to his nose. Give him a very small space to be able to breathe, then add a little more dirt.