Being deaf will prevent you from hearing things, with several sound-related messages no longer printing and others using an alternate message - this also includes verbal messages for seduction and charming. You cannot use stethoscopes while deaf.
You can end up deafened through the following methods:
- The monster-summoning effect of a magic trap.
- Falling unconscious from hunger.
- Applying a leather drum.
- In the Plane of Air, being caught inside a cloud during a storm will deafen you, among other undesired effects.
While deafness does not factor directly into any monster's attacks, most contextual messages hinting what is on your current floor will not appear, which may hinder you slightly. In addition, you will not hear any combat sounds either, which includes your pets fighting monsters or other monsters zapping wands.
In addition to simply going away over time, deafness is trivial enough to cure with a unicorn horn. Despite posing relatively little threat on its own, it should be cured as soon as possible in order to efficiently keep track of your surroundings - this can prevent unwelcome surprises from monsters that you would not have heard, e.g. a gelatinous cube eating and engulfing items.