Public domain, a fast open source speech processing toolkit written entirely in C++ and uses the ArrayFire tensor library and the flashlight machine learning library for maximum efficiency. [BSD]

Recurrent neural network for audio noise reduction. [BSD-3-Clause]

Single file audio playback and capture library. [Unlicense] website

A library for audio and music analysis.[GPL-3.0] website

C library for cross-platform real-time audio input and output. [MIT] website

Easy, portable audio engine for games. [zlib]

Public domain, header-only MP3 decoder with clean-room implementation. [CC0]

C++ Audio and Music DSP Library. [MIT]

C library with C++ wrapper for reading and writing files containing sampled sound through one standard library interface. [LGPL-2.1] website

Single file audio decoding libraries for C and C++. [Unlicense]

A simple C++ library for reading and writing audio files. [MIT]

Verovio is a fast and lightweight music notation engraving library. [LGPL] website

Easy and efficient audio synthesis in C++. [Unlicense]

C++ library for reading and writing audio files. [MIT]

SimplE Lossless Audio. [MIT]

A cross-platform audio engine designed with the needs of games in mind. [Apache-2.0] website

An easy to use crossplatform audio engine and audio content creation tool for games. [Free for non-commercial/Commercial]

Fast, modern C++ DSP framework, FFT, FIR/IIR filters, Sample Rate Conversion. [GPL/Proprietary]

LAME is a high quality MPEG Audio Layer III (MP3) encoder. [LGPL]

Open Audio Library - A crossplatform audio API. [BSD/LGPL/Proprietary]

A totally open, royalty-free, highly versatile audio codec. [BSD]

PortAudio is a free, cross-platform, open-source, audio I/O library. [MIT]

A free codec for free speech. Obsoleted by Opus. [BSD]

Ogg Vorbis is a fully open, non-proprietary, patent-and-royalty-free, general-purpose compressed audio format. [BSD]