FMOD_DSP_PITCHSHIFT doesn't work for mp3 on iOS

I was trying to implement a function to stretch the Sound speed ,without changing it's pitch and time scale. by documents and tips i can get from google. I try the method to set the frequency of channel to slow of fast the speed.then use FMOD_DSP_PITCHSHIFT to correct the pitch sounds as default;

i was using wav format sound file for test and build function.so far so good until i'm trying to intergrate product resource which sound file was encoded as MP3.awkwad situation occurred , PITCHSHIFT DSP doesn't work at MP3 sound channel. console log looks fine with no exception & error.Same project and setting everything works fine in iOS Simulator.

After some research and experiments, results indicates even m4a works fine at iOS. I use food examples to reproduce this situation. result is same as my project.

I wonder is this some kind of bug? or i missed something at configuration. sample code was based on FMOD Sample project Play stream.

`/*==============================================================================
Play Stream Example
Copyright (c), Firelight Technologies Pty, Ltd 2004-2015.

This example shows how to simply play a stream such as an MP3 or WAV. The stream
behaviour is achieved by specifying FMOD_CREATESTREAM in the call to 
System::createSound. This makes FMOD decode the file in realtime as it plays,
instead of loading it all at once which uses far less memory in exchange for a
small runtime CPU hit.
==============================================================================*/
#include "fmod.hpp"
#include "common.h"

int FMOD_Main()
{
    FMOD::System     *system;
    FMOD::Sound      *sound, *sound_to_play;
    FMOD::Channel    *channel = 0;
    FMOD_RESULT       result;
    FMOD::DSP        * pitch_shift;
    unsigned int      version;
    void             *extradriverdata = 0;
    int               numsubsounds;

    Common_Init(&extradriverdata);

    /*
        Create a System object and initialize.
    */
    result = FMOD::System_Create(&system);
    ERRCHECK(result);

    result = system->getVersion(&version);
    ERRCHECK(result);

    if (version < FMOD_VERSION)
    {
        Common_Fatal("FMOD lib version %08x doesn't match header version %08x", version, FMOD_VERSION);
    }

    result = system->init(32, FMOD_INIT_NORMAL, extradriverdata);
    ERRCHECK(result);

    result = system->createDSPByType(FMOD_DSP_TYPE_PITCHSHIFT, &pitch_shift);
    ERRCHECK(result);


    /*
        This example uses an FSB file, which is a preferred pack format for fmod containing multiple sounds.
        This could just as easily be exchanged with a wav/mp3/ogg file for example, but in this case you wouldnt need to call getSubSound.
        Because getNumSubSounds is called here the example would work with both types of sound file (packed vs single).
    */
    result = system->createSound(Common_MediaPath("aaa.m4a"), FMOD_LOOP_NORMAL | FMOD_2D, 0, &sound);
    ERRCHECK(result);

    result = sound->getNumSubSounds(&numsubsounds);
    ERRCHECK(result);

    if (numsubsounds)
    {
        sound->getSubSound(0, &sound_to_play);
        ERRCHECK(result);
    }
    else
    {
        sound_to_play = sound;
    }

    /*
        Play the sound.
    */
    result = system->playSound(sound_to_play, 0, false, &channel);
    ERRCHECK(result);
    result = channel->addDSP(0, pitch_shift);
    ERRCHECK(result);
            float        pitch = 1.f;
    result = pitch_shift->setParameterFloat(FMOD_DSP_PITCHSHIFT_PITCH, pitch);
    ERRCHECK(result);
    pitch_shift->setActive(true);
    ERRCHECK(result);
    float defaultFrequency;
    result = channel->getFrequency(&defaultFrequency);
    ERRCHECK(result);
    /*
        Main loop.
    */
    do
    {
        Common_Update();

        if (Common_BtnPress(BTN_ACTION1))
        {
            bool paused;
            result = channel->getPaused(&paused);
            ERRCHECK(result);
            result = channel->setPaused(!paused);
            ERRCHECK(result);
        }
        if (Common_BtnPress(BTN_DOWN)) {
            char valuestr;
            int valuestrlen;
            pitch_shift->getParameterFloat(FMOD_DSP_PITCHSHIFT_PITCH, &pitch, &valuestr, valuestrlen);
            pitch+=0.1f;
            pitch = pitch>2.0f?2.0f:pitch;
            pitch_shift->setParameterFloat(FMOD_DSP_PITCHSHIFT_PITCH, pitch);
            channel->setFrequency(defaultFrequency/pitch);

        }
        if (Common_BtnPress(BTN_UP)) {
            char valuestr;
            int valuestrlen;
            pitch_shift->getParameterFloat(FMOD_DSP_PITCHSHIFT_PITCH, &pitch, &valuestr, valuestrlen);
            pitch-=0.1f;
            pitch = pitch<0.5f?0.5f:pitch;
            pitch_shift->setParameterFloat(FMOD_DSP_PITCHSHIFT_PITCH, pitch);
            channel->setFrequency(defaultFrequency/pitch);
        }

        result = system->update();
        ERRCHECK(result);

        {
            unsigned int ms = 0;
            unsigned int lenms = 0;
            bool         playing = false;
            bool         paused = false;

            if (channel)
            {
                result = channel->isPlaying(&playing);
                if ((result != FMOD_OK) && (result != FMOD_ERR_INVALID_HANDLE))
                {
                    ERRCHECK(result);
                }

                result = channel->getPaused(&paused);
                if ((result != FMOD_OK) && (result != FMOD_ERR_INVALID_HANDLE))
                {
                    ERRCHECK(result);
                }

                result = channel->getPosition(&ms, FMOD_TIMEUNIT_MS);
                if ((result != FMOD_OK) && (result != FMOD_ERR_INVALID_HANDLE))
                {
                    ERRCHECK(result);
                }

                result = sound_to_play->getLength(&lenms, FMOD_TIMEUNIT_MS);
                if ((result != FMOD_OK) && (result != FMOD_ERR_INVALID_HANDLE))
                {
                    ERRCHECK(result);
                }
            }

            Common_Draw("==================================================");
            Common_Draw("Play Stream Example.");
            Common_Draw("Copyright (c) Firelight Technologies 2004-2015.");
            Common_Draw("==================================================");
            Common_Draw("");
            Common_Draw("Press %s to toggle pause", Common_BtnStr(BTN_ACTION1));
            Common_Draw("Press %s to quit", Common_BtnStr(BTN_QUIT));
            Common_Draw("");
            Common_Draw("Time %02d:%02d:%02d/%02d:%02d:%02d : %s", ms / 1000 / 60, ms / 1000 % 60, ms / 10 % 100, lenms / 1000 / 60, lenms / 1000 % 60, lenms / 10 % 100, paused ? "Paused " : playing ? "Playing" : "Stopped");
            Common_Draw("Pitch %02f",pitch);
        }

        Common_Sleep(50);
    } while (!Common_BtnPress(BTN_QUIT));

    /*
        Shut down
    */
    result = sound->release();  /* Release the parent, not the sound that was retrieved with getSubSound. */
    ERRCHECK(result);
    result = system->close();
    ERRCHECK(result);
    result = system->release();
    ERRCHECK(result);

    Common_Close();

    return 0;
}

`