My son with OCD also does this. He has (mostly) stopped this and his other outward symptoms under the care of a psychiatrist and therapist. Looking back it was one of the earliest signs. It is a form of compulsive reassurance seeking which is a very common symptom.

(Not saying this is definitely what’s going on with your son- just saying it resonated as an ocd symptom rather than a hearing issue)

Could be OCD.

Anxiety also causes you to miss a ton of what people are saying.