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, and assigning the audio object to be either a base object or an extension object based at least in part on said analyzing, 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 making the channels of audio available to a receiver together with the extension objects, thereby enabling the receiver to render the extension objects separately from the audio channels if the receiver is capable of doing so while still enabling the receiver to output the audio channels if the receiver is not capable of rendering the extension objects.