Hearing problems
I'm talking here about deafness or partial deafness. As a lot of the
content on the internet is visual deaf users are not as badly effected as
visually impaired users. If your web site has a lot of video or audio though
you need to make sure you provide captions for them so people who can't
get at your content can at least have a pretty good idea what is going on
in it. This can be difficult if you have a load of podcasts for example and
you could them consider providing transcripts of them.
Example
In this video below a caption has been used so a hearing impaired person will have a reasonable idea what the video clip is about.
"This video is by a man called Tom Lehrer in it he sings the names of all the elements in the periodic table to the song "Modern Major General" by Gillbert and Sullivan. Lehrer was a Mathematician and Musician an odd mix".