libKeyFinder can be used to estimate the musical key of digital recordings.

It is the basis of the KeyFinder GUI app, which is available as a binary download for Mac OSX and Windows at www.ibrahimshaath.co.uk/keyfinder

For the most basic use case, do something like this:

static KeyFinder::KeyFinder k;                         // static because it retains useful resources

KeyFinder::Parameters p;                               // it's usually best to use the default parameters
p.setHopSize(8192);                                    // but you can change stuff if you want

KeyFinder::AudioData a;                                // build an empty audio object

a.setFrameRate(yourAudioBuffer.framerate);             // for best results downsample your audio to 4410 Hz
a.setChannels(yourAudioBuffer.channels);               // KF will reduce to mono if you don't
a.addToSampleCount(yourAudioBuffer.length);            // prepare sufficient space in the audio object

while (int i = 0; i < yourAudioBuffer.length; i++) {
  a.setSample(i, yourAudioBuffer[i]);                  // copy your audio into the object
}

KeyFinder::KeyDetectionResult r =  k.findKey(a, p);    // run the analysis

doSomethingWith(r.globalKeyEstimate);                  // and do something with the result!