Setting volumes for multiple streams using Media Foundation - windows

Question

I'm providing audio code for an application that will have multiple streams of audio being played back at the same time. I'm a bit confused by all of the different options, and there are some specific things that I don't quite understand.
I am using the IAudioClient calls to get and set volumes. Is that the best way to get volumes for multiple streams?
It appears that I have to call IAudioClient::Initialize. This function requires a WAVEFORMATEX structure. Are any parameters from that other than the number of channels used in volume setting? Also, it appears that Initialize can only be used once, and volume setting and reading happens many times. Should I save the reference to the IAudioClient and use it each time, or can I release it each time I get or set a volume?
How do I differentiate between two streams being played on the same device (endpoint)?
Here's the code that sets the volume (with the usual checks to make sure each call succeeded eliminated to save space):
hr = CoCreateInstance(__uuidof(MMDeviceEnumerator), NULL, CLSCTX_INPROC_SERVER, IID_PPV_ARGS(&DeviceEnumerator));
hr = DeviceEnumerator->GetDevice((wchar_t *)currentPlaybackDevice.id, &pPlaybackDevice);
hr = pPlaybackDevice->Activate(__uuidof(IAudioClient), CLSCTX_INPROC_SERVER, NULL, reinterpret_cast<void **>(&pPlaybackClient));
hr = pPlaybackClient->Initialize(AUDCLNT_SHAREMODE_SHARED, 0, 0, 0, &pWaveFormat, 0);
hr = pPlaybackClient->GetService(__uuidof(IAudioStreamVolume), (void **)&pStreamVolume);
hr = pStreamVolume->GetChannelCount(&channels);
for(UINT32 i = 0; i < channels; i++)
chanVolumes[i] = playbackLevel;
hr = pStreamVolume->SetAllVolumes(channels, chanVolumes);

Answer 1

Number of channels is irrelevant to volume. T adjust volume you need to obtain interfaces IAudioStreamVolume, IChannelAudioVolume. See MSDN writes:
The IAudioStreamVolume interface enables a client to control and
monitor the volume levels for all of the channels in an audio stream.
The client obtains a reference to the IAudioStreamVolume interface on
a stream object by calling the IAudioClient::GetService method with
parameter riid set to REFIID IID_IAudioStreamVolume.
Here is the code snippet for you. It plays synthesized sine wave at louder volume for a few seconds, then continues with updated volume to keep playing quietly.
#define _USE_MATH_DEFINES
#include <math.h>
#include <mmdeviceapi.h>
#include <audioclient.h>
#define _A ATLASSERT
#define __C ATLENSURE_SUCCEEDED
#define __D ATLENSURE_THROW
int _tmain(int argc, _TCHAR* argv[])
{
__C(CoInitialize(NULL));
CComPtr<IMMDeviceEnumerator> pMmDeviceEnumerator;
__C(pMmDeviceEnumerator.CoCreateInstance(__uuidof(MMDeviceEnumerator)));
CComPtr<IMMDevice> pMmDevice;
__C(pMmDeviceEnumerator->GetDefaultAudioEndpoint(eRender, eMultimedia, &pMmDevice));
CComPtr<IAudioClient> pAudioClient;
__C(pMmDevice->Activate(__uuidof(IAudioClient), CLSCTX_ALL, NULL, (VOID**) &pAudioClient));
CComHeapPtr<WAVEFORMATEX> pWaveFormatEx;
__C(pAudioClient->GetMixFormat(&pWaveFormatEx));
static const REFERENCE_TIME g_nBufferTime = 60 * 1000 * 10000i64; // 1 minute
__C(pAudioClient->Initialize(AUDCLNT_SHAREMODE_SHARED, 0, g_nBufferTime, 0, pWaveFormatEx, NULL));
#pragma region Data
CComPtr<IAudioRenderClient> pAudioRenderClient;
__C(pAudioClient->GetService(__uuidof(IAudioRenderClient), (VOID**) &pAudioRenderClient));
UINT32 nSampleCount = (UINT32) (g_nBufferTime / (1000 * 10000i64) * pWaveFormatEx->nSamplesPerSec) / 2;
_A(pWaveFormatEx->wFormatTag == WAVE_FORMAT_EXTENSIBLE);
const WAVEFORMATEXTENSIBLE* pWaveFormatExtensible = (const WAVEFORMATEXTENSIBLE*) (const WAVEFORMATEX*) pWaveFormatEx;
_A(pWaveFormatExtensible->SubFormat == KSDATAFORMAT_SUBTYPE_IEEE_FLOAT);
// ASSU: Mixing format is IEEE Float PCM
BYTE* pnData = NULL;
__C(pAudioRenderClient->GetBuffer(nSampleCount, &pnData));
FLOAT* pfFloatData = (FLOAT*) pnData;
for(UINT32 nSampleIndex = 0; nSampleIndex < nSampleCount; nSampleIndex++)
for(WORD nChannelIndex = 0; nChannelIndex < pWaveFormatEx->nChannels; nChannelIndex++)
pfFloatData[nSampleIndex * pWaveFormatEx->nChannels + nChannelIndex] = sin(1000.0f * nSampleIndex / pWaveFormatEx->nSamplesPerSec * 2 * M_PI);
__C(pAudioRenderClient->ReleaseBuffer(nSampleCount, 0));
#pragma endregion
CComPtr<ISimpleAudioVolume> pSimpleAudioVolume;
__C(pAudioClient->GetService(__uuidof(ISimpleAudioVolume), (VOID**) &pSimpleAudioVolume));
__C(pSimpleAudioVolume->SetMasterVolume(0.50f, NULL));
_tprintf(_T("Playing Loud\n"));
__C(pAudioClient->Start());
Sleep(5 * 1000);
_tprintf(_T("Playing Quiet\n"));
__C(pSimpleAudioVolume->SetMasterVolume(0.10f, NULL));
Sleep(15 * 1000);
// NOTE: We don't care for termination crash
return 0;
}