Though not so common at this age group, You need to rule out Auditory Hallucinations
Visit a child psychologist/ child psychiatrist
If its persistent, he has to be evaluated surely for this.
How were his developmental milestones- normal or delayed? How is he at school now? Is he mingling with his friends as usual? Any chance of being bullied or abused ?
These things have to be looked into when you visit your child psychologist.
Hope my answer helps you.