The ears are very much related to balance and dizziness.
Balance is maintained not only by the vestibular system found in your ears but also by your visual and sensory systems. If any one of these systems is damaged, you may experience dizziness or loss of balance.
Yet I'm not finding anything out about the role of deafness in 5E combat particularly regarding melee.
I know if I were DM'ing I'd probably give a negative to hit for Strength / 2handed weapon use, and full disadvantage for finesse weapons. Or maybe to simplify things give a percentage roll with like a 20% chance of falling down on a missed attack… something anyways to show the character is having issues.
But are there any Rules or Guidelines regarding how deafness affects melee combat?
Best Answer
Deafened is a specific Condition called out in page 290 of the PHB:
Given that spells only do what they say they do and nothing more (I'm assuming the Deafened condition was cause by a spell), that appears to be all. By RAW, there are no balancing issues, no penalties to hit or anything like that.
There are rare adverse effects such as they cannot benefit from Bardic Inspiration.
There are even rarer benefits as well, such as they won't be affected by spells that require them to hear, like Dissonant Whispers (strangely, Command does not need a creature to hear).
In summary, becoming deafened isn't very threatening; it's much better to blind a melee combatant, instead.