Sample code from apple iPhoneMultichannelMixerTest can't run mp3 file

Question

Hi i want to control channel volume from my app and iPhoneMultichannelMixerTest (http://developer.apple.com/iphone/library/samplecode/iPhoneMultichannelMixerTest/)sample code form apple looks perfect for me. but i m heaving problem in playing mp3 files. code crashes in MultichannelMixerController.m at

static OSStatus renderInput(void
*inRefCon, AudioUnitRenderActionFlags *ioActionFlags, const AudioTimeStamp *inTimeStamp, UInt32 inBusNumber, UInt32 inNumberFrames, AudioBufferList
*ioData) {
    SoundBufferPtr sndbuf = (SoundBufferPtr)inRefCon;

    UInt32 bufSamples = sndbuf[inBusNumber].numFrames;  AudioUnitSampleType *in = sndbuf[inBusNumber].data;    AudioUnitSampleType *outA = (AudioUnitSampleType
*)ioData->mBuffers[0].mData;  AudioUnitSampleType *outB = (AudioUnitSampleType
*)ioData->mBuffers[1].mData;

    UInt32 sample = sndbuf[inBusNumber].sampleNum;  for (UInt32 i = 0; i < inNumberFrames;
++i) {
        if (1 == inBusNumber) {
            outA[i] = 0;
            outB[i] = in[sample++];    //crash on this line.
        } else {
             outA[i] = in[sample++];
             outB[i] = 0;
        }
        if (sample >= bufSamples) sample = 0;
    }
    sndbuf[inBusNumber].sampleNum = sample;    // printf("bus %d sample %d\n", inBusNumber, sample);
      return noErr; }

can any one tell me why this is happening. or is there any limitation to play mp3 on iPhone???

Thankz in Adv.

Usman