1. A method of encoding object-based audio, the method comprising: for each audio object of a plurality of audio objects: accessing the audio object, the audio object comprising attribute metadata and audio signal data, analyzing one or both of the attribute metadata and the audio signal data with respect to one or more object selection rules, the one or more object selection rules relating to at least a movement characteristic of the audio object reflected in the attribute metadata, and assigning the audio object to be either a base object or an extension object based at least in part on said analyzing, said assigning comprising assigning the audio object to be an extension object in response to determining that the movement characteristic reflected in the attribute metadata indicates at least one of: a speed of the audio object exceeding a speed threshold or a duration of movement of the audio object exceeding a duration threshold, wherein a first number of the audio objects are assigned to be base objects and a second number of the audio objects are assigned to be extension objects; rendering the base objects and the extension objects to produce channels of audio; and transmitting or causing transmission of the channels of audio to a receiver together with the extension objects, thereby enabling the receiver to render for playback the extension objects separately from the audio channels if the receiver is capable of doing so while still enabling the receiver to playback the audio channels if the receiver is not capable of rendering the extension objects, wherein the method is performed by one or more hardware processors.