libopusfile 0.12 (The header files)

opusfile provides a high-level API for decoding and seeking within .opus files.
It includes:
* Support for all files with at least one Opus stream
  (including multichannel files or Ogg files where Opus is muxed with something else).
* Full support, including seeking, for chained files.
* A simple stereo downmixing API
  (allowing chained files to be decoded with a single output format, even if the channel count
changes).
* Support for reading from a file, memory buffer, or over HTTP(S) (including seeking).
* Support for both random access and streaming data sources.

https://www.opus-codec.org/