JUCE!

Qt Creator is a cross-platform C++, JavaScript and QML integrated development environment which is part of the SDK for the Qt Framework.

Nana C++ Library takes aim at easy-to-use and portable library, it provides a GUI framework and threads for easy programming with modern C++ methods, such as traits, metaprogramming and other template technologies. http://sourceforge.net/p/nanapro/blog

PortAudio is a cross platform, open-source, audio I/O library. It provides a very simple API for recording and/or playing sound using a simple callback function.

The FOX toolkit is an open source, cross-platform widget toolkit, that is, a library of basic elements for building a graphical user interface (GUI).

RtAudio is a set of C++ classes that provide a common API (Application Programming Interface) for realtime audio input/output across Linux, Macintosh OS-X and Windows operating systems. RtAudio significantly simplifies the process of interacting with computer audio hardware. It was designed with the following objectives: