Getting output of Async SAPI call? - c++

Question

I have a working example using the Windows text-to-speech API, but the problem is it freezes when given a large amount of text.
To solve this I want to make the process asynchronous. I've found the SpeechVoiceSpeakFlags::SVSFlagsAsync flag, but how do I get the results of a previously-submitted request?
Here is my code:
char* TextToWavInner( const wchar_t* voiceRequiredAttributes, const wchar_t* voiceOptionalAttributes, long rate, const wchar_t* textToRender, ULONG* pBytesRead )
{
HRESULT hr;
CComPtr<ISpVoice> cpVoice; //Will send data to ISpStream
CComPtr<ISpStream> cpStream; //Will contain IStream
CComPtr<IStream> cpBaseStream; //raw data
ISpObjectToken* cpToken( NULL ); //Will set voice characteristics
GUID guidFormat;
WAVEFORMATEX* pWavFormatEx = nullptr;
hr = cpVoice.CoCreateInstance( CLSID_SpVoice );
if ( FAILED( hr ) )
return NULL;
hr = SpFindBestToken( SPCAT_VOICES, voiceRequiredAttributes, voiceOptionalAttributes, &cpToken );
if ( FAILED( hr ) )
return NULL;
hr = cpVoice->SetVoice( cpToken );
cpToken->Release();
if ( FAILED( hr ) )
return NULL;
cpVoice->SetRate( rate );
hr = cpStream.CoCreateInstance( CLSID_SpStream );
if ( FAILED( hr ) )
return NULL;
hr = CreateStreamOnHGlobal( NULL, true, &cpBaseStream );
if ( FAILED( hr ) )
return NULL;
hr = SpConvertStreamFormatEnum( SPSF_44kHz16BitMono, &guidFormat, &pWavFormatEx );
if ( FAILED( hr ) )
return NULL;
hr = cpStream->SetBaseStream( cpBaseStream, guidFormat, pWavFormatEx );
if ( FAILED( hr ) )
return NULL;
hr = cpVoice->SetOutput( cpStream, false );
if ( FAILED( hr ) )
return NULL;
SpeechVoiceSpeakFlags voiceFlags = ( SpeechVoiceSpeakFlags ) ( SpeechVoiceSpeakFlags::SVSFlagsAsync | SpeechVoiceSpeakFlags::SVSFPurgeBeforeSpeak );
hr = cpVoice->Speak( textToRender, voiceFlags, NULL );
if ( FAILED( hr ) )
return NULL;
LARGE_INTEGER a = { 0 };
hr = cpStream->Seek( a, STREAM_SEEK_SET, NULL );
if ( FAILED( hr ) )
return NULL;
STATSTG stats;
cpStream->Stat( &stats, STATFLAG_NONAME );
ULONG sSize = stats.cbSize.LowPart;
char* pBuffer = new char[ sSize ];
cpStream->Read( pBuffer, sSize, pBytesRead );
return pBuffer;
}