Every time a packet of audio (the payload) is sent from a device, it includes information that assists the network in moving the packet around as well as information that the receiving device needs in order to know how to handle the payload. It's not unlike writing a letter to someone: The letter is the payload, and the envelope it's mailed in is the information the postal service needs to get your letter to the receipt.

This information is tacked on to the beginning of the transmitted packet in the form of headers.