Welcome to HealthcareMagic.
I would ask you to tell me since how long have you been experiencing these voices.
This is a situation which needs thorough evaluation from a Psychiatrist
.Nothing can be clarified with this history.
To give you an idea , Sleep is in various stages.Meaning when you go to sleep , when you reach deep sleep , the journey is in various stages where brain resonates with various frequency waves.Due to these variations there can be a chance of audio heard like this.
This might be a type of Hypnogogic disturbances.
I suggest you to visit your Psychiatrist for complete evaluation.