See also: "Essentia is an open-source C++ library for audio analysis and audio-based music information retrieval released under the Affero GPLv3 license (also available under proprietary license upon request). It contains an extensive collection of reusable algorithms which implement audio input/output functionality, standard digital signal processing blocks, statistical characterization of data, and a large set of spectral, temporal, tonal and high-level music descriptors. In addition, Essentia can be complemented with Gaia, a C++ library with python bindings which implement similarity measures and classifications on the results of audio analysis, and generate classification models that Essentia can use to compute high-level description of music (same license terms apply)."
http://essentia.upf.edu/
(which I discovered from the game Crypt of the Necrodancer which uses it to do (at least?) beat detection for your custom music)