KFR is described as 'is built for developers who need a powerful toolkit for audio and DSP applications. The framework is packed with ready-to-use C++ classes and functions for various DSP tasks from high-quality filtering to small helpers to improve development speed' and is an app in the Development category. There are four alternatives to KFR for Mac, Windows and Linux. The best alternative is JUCE. It's not free, so if you're looking for a free alternative, you could try PortAudio or RtAudio.
RtAudio is a set of C++ classes that provide a common API 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.