Opus is a totally open, royalty-free, highly versatile audio codec. Opus is
unmatched for interactive speech and music transmission over the Internet, but
also intended for storage and streaming applications. 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.

Технологии

Opus can handle a wide range of audio applications, including Voice over IP,
videoconferencing, in-game chat, and even remote live music performances. It
can scale from low bit-rate narrowband speech to very high quality stereo
music. Supported features are:

Bit-rates from 6 kb/s to 510 kb/s
Частота дискретизации от 8 кГц (узкая полоса) до 48 кГц (полная полоса)
Размеры кадра от 2,5 мс до 60 мс
Support for both constant bit-rate (CBR) and variable bit-rate (VBR)
Audio bandwidth from narrowband to full-band
Support for speech and music
Support for mono and stereo
Support for up to 255 channels (multistream frames)
Dynamically adjustable bitrate, audio bandwidth, and frame size
Хорошая устойчивость к потерям и скрытие пакетов (ПЛК)
Floating point and fixed-point implementation

You can read the full specification, including the reference implementation,
in RFC 6716.