This is currently really only applicable to Opus-encoded files. It assumes
AmbiX (SN3D normalization, ACN ordering) and only comes into play when the
channel layout is blank. FFmpeg/libavcodec doesn't have a way to detect
B-Format input or what normalization and ordering it uses. Note in particular
.amb files do not play correctly (libavcodec seems to apply a default channel
layout for 4-channel wav-type files, regardless of its channel mask value).
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