Opus is a totally open, royalty-free, highly versatile audio codec.
It is standardized by the Internet Engineering Task Force (IETF)
as RFC 6716 which incorporated technology from Skype's SILK codec
and Xiph.Org's CELT codec.

Opus is designed to handle a wide range of interactive audio
applications, including Voice over IP, videoconferencing, in-game
chat, and even live, distributed music performances. It scales from
low bitrate narrowband speech at 6 kbit/s to very high quality
stereo music at 510 kbit/s. Opus uses both Linear Prediction (LP)
and the Modified Discrete Cosine Transform (MDCT) to achieve good
compression of both speech and music.