Determine if windows is currently playing sound

Here is a sample C# code that determines if Windows is rendering any audio stream. It uses Windows Core Audio API (specifically the IAudioMeterInformation interface) and is supported on Vista and higher.

public static bool IsWindowsPlayingSound()
{
    var enumerator = (IMMDeviceEnumerator)(new MMDeviceEnumerator());
    var speakers = enumerator.GetDefaultAudioEndpoint(EDataFlow.eRender, ERole.eMultimedia);
    var meter = (IAudioMeterInformation)speakers.Activate(typeof(IAudioMeterInformation).GUID, 0, IntPtr.Zero);
    var value = meter.GetPeakValue();

    // this is a bit tricky. 0 is the official "no sound" value
    // but for example, if you open a video and plays/stops with it (w/o killing the app/window/stream),
    // the value will not be zero, but something really small (around 1E-09)
    // so, depending on your context, it is up to you to decide
    // if you want to test for 0 or for a small value
    return value > 1E-08;
}

[ComImport, Guid("BCDE0395-E52F-467C-8E3D-C4579291692E")]
private class MMDeviceEnumerator
{
}

private enum EDataFlow
{
    eRender,
    eCapture,
    eAll,
}

private enum ERole
{
    eConsole,
    eMultimedia,
    eCommunications,
}

[InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("A95664D2-9614-4F35-A746-DE8DB63617E6")]
private interface IMMDeviceEnumerator
{
    void NotNeeded();
    IMMDevice GetDefaultAudioEndpoint(EDataFlow dataFlow, ERole role);
    // the rest is not defined/needed
}

[InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("D666063F-1587-4E43-81F1-B948E807363F")]
private interface IMMDevice
{
    [return: MarshalAs(UnmanagedType.IUnknown)]
    object Activate([MarshalAs(UnmanagedType.LPStruct)] Guid iid, int dwClsCtx, IntPtr pActivationParams);
    // the rest is not defined/needed
}

[InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("C02216F6-8C67-4B5B-9D00-D008E73E0064")]
private interface IAudioMeterInformation
{
    float GetPeakValue();
    // the rest is not defined/needed
}

As said in my comment, I have also created an open source c++ project, a simple friction-free zero-dependencies console application, available here: https://github.com/smourier/IsWindowsPlayingSound.
I have added one x86 release binary that should support 32 and 64 bit OSes: https://github.com/smourier/IsWindowsPlayingSound/releases

You can use it in PowerShell like any external .exe program. It will return an error level that you can retrieve using standard ways, for example: https://blogs.msdn.microsoft.com/powershell/2006/09/15/errorlevel-equivalent/

Here is the equivalent C++ code:

  #include "stdafx.h" // includes <Endpointvolume.h> and <Mmdeviceapi.h>

  #define WIDEN2(x) L ## x
  #define WIDEN(x) WIDEN2(x)
  #define __WFILE__ WIDEN(__FILE__)
  #define HRCHECK(__expr) {hr=(__expr);if(FAILED(hr)){wprintf(L"FAILURE 0x%08X (%i)\n\tline: %u file: '%s'\n\texpr: '" WIDEN(#__expr) L"'\n",hr, hr, __LINE__,__WFILE__);goto cleanup;}}
  #define RELEASE(__p) {if(__p!=nullptr){__p->Release();__p=nullptr;}}

  int main(int argc, char *argv[])
  {
    BOOL playing = FALSE;
    BOOL loopmode = FALSE;
    float epsilon = 1E-07;
    float value = 0;
    HRESULT hr = S_OK;
    IMMDeviceEnumerator* pEnumerator = NULL;
    IMMDevice *pDevice = NULL;
    IAudioMeterInformation *pMeter = NULL;

    // Parse optional args
    // "loop" -> sets a loop mode for easy testing
    // <float value> -> changes epsilon
    for (int i = 1; i < argc; i++)
    {
      if (!strcmp(argv[i], "loop"))
      {
        loopmode = TRUE;
        continue;
      }

      float eps = atof(argv[i]);
      if (eps != 0.0)
      {
        epsilon = eps;
        continue;
      }
    }

    CoInitialize(NULL);
    HRCHECK(CoCreateInstance(__uuidof(MMDeviceEnumerator), NULL, CLSCTX_ALL, __uuidof(IMMDeviceEnumerator), (void**)&pEnumerator));
    HRCHECK(pEnumerator->GetDefaultAudioEndpoint(EDataFlow::eRender, ERole::eMultimedia, &pDevice));
    HRCHECK(pDevice->Activate(__uuidof(IAudioMeterInformation), CLSCTX_ALL, NULL, (void**)&pMeter));
    do
    {
      HRCHECK(pMeter->GetPeakValue(&value));
      playing = value > epsilon;
      if (!loopmode)
        break;

      printf("%.10f playing:%i\n", value, playing);
      Sleep(100);
    } while (TRUE);

  cleanup:
    RELEASE(pMeter);
    RELEASE(pDevice);
    RELEASE(pEnumerator);
    CoUninitialize();
    if (FAILED(hr))
    {
      printf("An error occurred: 0x%08X\n", hr);
      return hr;
    }

    if (playing)
    {
      printf("Windows is playing a sound.\n");
    }
    else
    {
      printf("Windows is not playing a sound.\n");
    }
    return playing;
  }