CLAM (audio software)
CLAM (C++ Library for Audio and Music) is an open-source framework for research and application development in the audio and music domain. It is based on the concept of data-processing modules linked into a network. Modules can perform complex audio signal analysis, transformations and synthesis. CLAM also provides a uniform interface to common tasks within audio applications, such as accessing audio devices and audio files. CLAM serves as a library for C++ application development, but a graphical interface also allows full applications to be built without coding. It won the 2006 ACM Multimedia Open Source Competition. References 

Audio libraries
C++ libraries
Software using the GPL license