This is a draft encapsulation guide for Opus audio in the mp4 (ISO Base) media container.
MP4 already has support for declaring encoder delay and pre-roll.
For pre-roll I believe we can use 'AudioRollRecoveryEntry' for pre-roll.
There's some work on codec-independent channel mapping, downmix and dynamic range control as part of ISO 14496-12 Amd4 http://mpeg.chiariglione.org/standards/mpeg-4/iso-base-media-file-format/text-isoiec-14496-122012-pdam-4-enhanced-audio-support We might be able use some of that, but it doesn't support the Opus case of needing to indicated which streams are coupled pairs. We'll still need to define our own extension for this.
Question: Better to reuse the channel mapping header entirely, or just report the coupled stream count and use the downmix table to do the mapping?