I got to an answer of my problem.
SyncPoint.Synchronize function is somehow not working in this case. So PXCMAudio output has some nullpoint parameters.

I jsut fixed some source code in pxcmvoice.h and pxcmvoice.cpp. But this is not async function at all.

in pxcmvoice.h line 116
from:

pxcmStatus ProcessAudioAsync(pxcmUID id, PXCMAudio ^%audio, PXCMScheduler::SyncPoint ^%sp);

to:

pxcmStatus ProcessAudioAsync(pxcmUID id, [Out] PXCMAudio ^%audio, [Out] PXCMScheduler::SyncPoint ^%sp);

in pxcmvoice.cpp line 151

from:

pxcmStatus PXCMVoiceSynthesis::ProcessAudioAsync(pxcmUID id, PXCMAudio ^%audio, PXCMScheduler::SyncPoint ^%sp) {

    PXCScheduler::SyncPoint *sp2=0;

    PXCAudio *audio2=0;

    sp=nullptr; audio=nullptr;

    pxcStatus sts=QueryInstance()->ProcessAudioAsync((pxcUID)id,&audio2,&sp2);

    if (sts>=PXC_STATUS_NO_ERROR) {

        audio=gcnew PXCMAudio(audio2,true);

        sp=gcnew PXCMScheduler::SyncPoint(sp2,true);

    }

    return (pxcmStatus)sts;

}

to:

pxcmStatus PXCMVoiceSynthesis::ProcessAudioAsync(pxcmUID id, [Out] PXCMAudio ^%audio, [Out] PXCMScheduler::SyncPoint ^%sp) {

    PXCAudio *audio2=0;

    PXCScheduler::SyncPoint *sp2=0;

    sp=nullptr; audio=nullptr;

    pxcStatus sts=QueryInstance()->ProcessAudioAsync((pxcUID)id,&audio2,&sp2);

    sp2->Synchronize(); // if not sync, audio2 may be null but sts is no error. And cannot synchronize outside of this function.

    if (sts>=PXC_STATUS_NO_ERROR) {

        audio=gcnew PXCMAudio(audio2,true);

        sp=gcnew PXCMScheduler::SyncPoint(sp2,true);

    }

    return (pxcmStatus)sts;

}

Maybe, this fix is not suitable because I read a little bit source code of this SDK.
So if there is wrong fix, please let me know.

@Xintian
Thank you for the reply.
I understand current status.
I wait for the next release :)