A monster is deaf when its deaf flag (as THINGS lump 3 or Hexen flag 3) is set. Monsters are normally alerted when they hear the player attack, even with the silent fist. Deaf monsters are not alerted by player sounds and remain dormant until they see the player or receive damage.
Deaf monsters are often placed just inside doorways or around corners to ambush unsuspecting players. The player may have cleared the room of all visible enemies, but the deaf monsters will still be lying in wait since they didn't hear the preceding firefight.