How do I encode into an MPEG-4 Structured Audio format?
You can’t, today. The techniques required for automatically producing a Structured Audio bitstream from an arbitrary sound are beyond today’s state of the art, although they are an active research topic. These techniques are often called “automatic source separation” or “automatic transcription”; there are many references within the audio processing literature on the capabilities of today’s methods. In the mean time, content authors will use special content creation tools to directly create Structured Audio bitstreams. This is not a fundamental obstacle to the use of MPEG-4 Structured Audio, because these tools are very similar to the ones that content authors use already; all that is required is to make them capable of producing MPEG-4 output bitstreams.
