Turtles can hear, however they do not have an outer ear that sticks out from their head as most animals do. Rather, they are equipped with inner ear mechanisms as well as auditory nerve and brain centers that are required for hearing.
The outer ear gathers sound vibrations to make them louder. It is for this reason that turtles do not hear airborne sounds as loudly as we humans can. However they do have a keen sense and can interpret vibrations within their environments. Hearing is not very important to turtles however, because their senses of vision and smell are excellent. What they lack in one area of the brain center, they definitely make up for elsewhere.