You can wield a tooled horn to use it as an ear trumpet, temporarily alleviating deafness for as long as you keep it wielded.

You can wear a second towel on top of an existing one. This makes you deaf as well as blind.

When the game calls for a message to be shown based on its string id in the quest.lua data file, it also supports running a function that tests whether the hero is blind and/or deaf and returns different string messages accordingly. (E.g. a lot of quest messages describe the surroundings, which doesn’t make sense if you’re blind.)

New artifact, the Ears of the Underworld. They enable you to hear even if you’re deaf and block all sound-based attacks.

Deafness multiplies your spell failure rate by a constant amount, due to maybe not getting the pronunciation right because you can’t hear yourself.

Deafness should provide total protection against cockatrices’ hissing attacks.

Deafness protects against cockatrice hissing and therefore their stoning. (This might be too powerful; it might need to just reduce the chance of stoning rather than eliminating it altogether.)