This is rather self explanatory.  If in the 'who' list you see {deaf} next
to a player's name, it means they are sleeping, in a soundproof room, or
a number of other reasons.  Shouting, telling nor yelling will reach them.
Your only hope is if you are Grouped with them, and can use Gtell. 

See Also: Who, Grouping, Gtell, Yell