Link to Paranoialmaniac's draft
yusuke says, "ISO and Apple recommend the use of edit list for removing priming samples from the presentation." libavformat's demuxer supports *one* edit list entry.
There's some work on codec-independent channel mapping, downmix and dynamic range control as part of [http://mpeg.chiariglione.org/standards/mpeg-4/iso-base-media-file-format/text-isoiec-14496-122012-pdam-4-enhanced-audio-support ISO 14496-12 Amd4] 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.