Naively sum a non-interleaved buffer of stereo, floating-point audio to mono using the Accelerate framework. See http://dsp.stackexchange.com/a/2485 for suggestion of a more sophisticated approach.

monoSumming.c

@import Accelerate;

static float* monoDataFromStereoNonInterleaved(const float *stereoData, size_t length) {
    vDSP_Length numberOfSamples = length / sizeof(float) / 2;
    const float *leftChannel = stereoData;
    const float *rightChannel = leftChannel + numberOfSamples;
    size_t monoDataLength = length / 2;
    float *monoData = malloc(monoDataLength);
    vDSP_vadd(leftChannel, 1, rightChannel, 1, monoData, 1, numberOfSamples);
    float scale = 0.5;
    vDSP_vsmul(monoData, 1, &scale, monoData, 1, numberOfSamples);
    return monoData;
}