Benefits of Deafness
The ability to lip-read would give the player the ability to spy on conversations that are out of earshot.
Note, this is not a direct benefit of deafness, as not all deaf characters can lip read and you don't need to be deaf to learn to lip read. In fact, the ability to lip read is one of the benefits of the Observant feat
A key benefit is that there are a number of monster effects which only take effect if the player character can't hear them, some examples are below (not a complete list):
Gibbering. The mouther babbles incoherently while it can see any creature and isn't incapacitated. Each creature that starts its turn within 20 feet of the mouther and can hear the gibbering must succeed on a DC 10 Wisdom saving throw. On a failure, the creature can't take reactions until the start of its next turn and rolls a d8 to determine what it does during its turn. On a 1 to 4, the creature does nothing. On a 5 or 6, the creature takes no action or bonus action and uses all its movement to move in a randomly determined direction. On a 7 or 8, the creature makes a melee attack against a randomly determined creature within its reach or does nothing if it can't make such an attack.
Luring Song. The harpy sings a magical melody. Every humanoid and giant within 300 feet of the harpy that can hear the song must succeed on a DC 11 Wisdom saving throw or be charmed until the song ends. The harpy must take a bonus action on its subsequent turns to continue singing. It can stop singing at any time. The song ends if the harpy is incapacitated.
While charmed by the harpy, a target is incapacitated and ignores the songs of other harpies. If the charmed target is more than 5 feet away from the harpy, the target must move on its turn toward the harpy by the most direct route, trying to get within 5 feet. It doesn't avoid opportunity attacks, but before moving into damaging terrain, such as lava or a pit, and whenever it takes damage from a source other than the harpy, the target can repeat the saving throw. A charmed target can also repeat the saving throw at the end of each of its turns. If the saving throw is successful, the effect ends on it.
A target that successfully saves is immune to this harpy's song for the next 24 hours.
Roar (3/Day). The sphinx emits a magical roar. Each time it roars before finishing a long rest, the roar is louder and the effect is different, as detailed below. Each creature within 500 feet of the sphinx and able to hear the roar must make a saving throw.
First Roar. Each creature that fails a DC 18 Wisdom saving throw is frightened for 1 minute. A frightened creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
Second Roar. Each creature that fails a DC 18 Wisdom saving throw is deafened and frightened for 1 minute. A frightened creature is paralyzed and can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
Third Roar. Each creature makes a DC 18 Constitution saving throw. On a failed save, a creature takes 44 (8d10) thunder damage and is knocked prone. On a successful save, the creature takes half as much damage and isn't knocked prone.
Wail (1/Day). The banshee releases a mournful wail, provided that she isn’t in sunlight. This wail has no effect on constructs and undead. All other creatures within 30 feet of her that can hear her must make a DC 13 Constitution saving throw. On a failure, a creature drops to 0 hit points. On a success, a creature takes 10 (3d6) psychic damage.
There are a number of spell effects which require the player to be able to hear for them to have effect. Some examples are below (again, this is not a comprehensive list). Note that this seems to be more common for Bard spells.
Choose a target within range that you can see. If it can hear you, it must succeed on a Wisdom save or take 1d4 psychic damage and have disadvantage on the next attack roll it makes before the end of its next turn.
This spell's damage increases by 1d4 when you reach 5th, 11th, and 17th level.
The target must make a Wisdom saving throw. On a failed save, it takes 3d6 psychic damage and must immediately use its reaction to move as far as its speed allows away from you. The creature doesn't move into obviously dangerous ground. On a successful save, the target takes half as much damage and doesn't have to move. A deafened creature automatically succeeds.
At Higher Levels: The damage increases by 1d6 for each slot level above 1st.
You cause visible creatures of your choice that can hear you to make a Wisdom saving throw. Any creature that can't be charmed is immune to this spell. If you or your companions are fighting the target, it has advantage. On a failed save, the target has disadvantage on Wisdom (Perception) checks made to perceive any creature other than you until the spell ends or until the target can no longer hear you. The spell ends if you are incapacitated or can no longer speak.
Disadvantages of Deafness
If the character is travelling in the wild they would be automatically fail any perception check to detect the sound of monsters such as the howling of a pack of wolves, the sound of footsteps behind the party, the rustling of an Owlbear in the bushes. If this character is on guard duty, there is a far greater chance they will be successfully ambushed by monsters.
Other types of Communication
While the deaf character is able to lip read in normal conversation, there are other situations where they would struggle:
- In the middle of combat (not hearing a warning shout from an ally)
- Trying to eavesdrop on a conversation where you can't see speakers
- If the speaker has their mouth covered (could be a fun bandit encounter where they are wearing masks and the player can't hear them!)
When vision is impaired
If the players vision is impaired, they will be at a serious disadvantage versus the others players who could rely on their hearing. Example situations would be:
- If the players are in the area of a Darkness spell
- In a fight against a creature with the ability to turn invisible (e.g. Invisible Stalker)
- In an area with dim or dark light (the torches have run out in a dungeon)
Beneficial Player Effects
There are a number of beneficial player effects which require hearing. This is especially common in Bard abilities.
Bard - Bardic Inspiration
You can use a bonus action on your turn to choose one creature other than yourself within 60 feet of you who can hear you. That creature gains one Bardic Inspiration die, a d6.
Bard - Song of Rest
Beginning at 2nd level, if you or any friendly creatures who can hear you make a performance regain hit points by spending hit dice at the end of the short rest, each of those creatures regains an extra 1d6 hit points.
Bard - Countercharm
At 6th level, as an action, you can start a performance that lasts until the end of your next turn. During that time, you and any friendly creatures within 30 feet of you have advantage on saving throws against being frightened or charmed. A creature must be able to hear you to gain this benefit. The performance ends early if you are incapacitated or silenced or if you voluntarily end it (no action required).
Fighter - Commander's Strike
Commander's Strike: When you take the Attack action on your turn, you can forgo one of your attacks and use a bonus action to direct one of your companions to strike. When you do so, choose a friendly creature who can see or hear you and expend one superiority die. That creature can immediately use its reaction to make one weapon attack, adding the superiority die to the attack's damage roll.
These are just a few thoughts from me and I'm sure I have missed many other benefits or disadvantages of the Deaf character. Hope that this helps!