They do have hearing students. Some are specially recruited. Some are specifically studying deafness-related subjects.

See -- http://www.gallaudet.edu/undergraduate_admissions/hearing_students/apply_as_a_hearing_undergraduate_student.html

At the very least, I'd think you need to be fluent in ASL.

I know Gallaudet University admits some hearing students each year, as it wants the hearing impaired to interact with those with normal hearing. Does anyone know how this works in practice? What the admission criteria are? How many are admitted?