One of the voices is that of the Approach controller who will be present when it is particularly busy or, for various reasons, the frequency for BNN/LAM is combined with the frequency for OCK/BIG. He/she will answer your first call e.g if holding is required or to issue descent clearance if you are already in the hold. This controller also liaises with adjacent sectors and can fine tune the approach sequence to minimize wake separation on final approach.
The other voice is the radar controller, known as the Intermediate Director, who will issue all the heading/speed instructions.