A lot of people think that being deaf means a person can't hear anything at all, but there are different levels of hearing loss, meaning each deaf or hard-of-hearing individual experiences a different amount of sound. Even some profoundly deaf people can hear very loud sounds or is exposed to sound through vibrations. Additionally, today's rapidly advancing technology-- including hearing aids, FM systems, and cochlear implants-- is providing deaf people with increasing access to sound.
However, even when using assistive technology many deaf and hard-of-hearing people report being able to hear but not understand sounds and speech. This is because cochlear hair cells, the cells that are damaged or missing in those with sensorineural hearing loss, are also in charge of the preliminary processing and filtering of sound, so hearing amplification technology doesn't always provide accurate sound access. Hearing can be likened to a radio station; if the station is staticy turning up the volume won't make it much clearer, it will just make louder static.
To listen to an example of what mild, moderate, and severe hearing loss sounds like, take a look at last year's post: What Does Deafness Sound Like?