Please explain to me how do you understand sound if you are not hearing it?
Most deaf people can hear sounds depends on how loud those sounds can be, some can understand the words, some cannot.
Each person's hearing loss is different, some with hearing loss can hear some sounds, some can hear sounds at a lower frequency better than at a higher frequency. You cannot say no deaf can understand sounds when they can't hear it, how do u know what they can hear and what they cannot?
Oral approach is for anyone, end of the discussion.