VolumeMixer.cs

using System;
using System.Runtime.InteropServices;

public class VolumeMixer
{
    [DllImport("user32.dll")]
    public static extern IntPtr FindWindow(string strClassName, string strWindowName);

    [DllImport("user32.dll", SetLastError = true)]
    public static extern uint GetWindowThreadProcessId(IntPtr hWnd, out uint processId);

    public static float? GetApplicationVolume(int pid)
    {
        ISimpleAudioVolume volume = GetVolumeObject(pid);
        if (volume == null)
            return null;

        float level;
        volume.GetMasterVolume(out level);
        Marshal.ReleaseComObject(volume);
        return level * 100;
    }

    public static bool? GetApplicationMute(int pid)
    {
        ISimpleAudioVolume volume = GetVolumeObject(pid);
        if (volume == null)
            return null;

        bool mute;
        volume.GetMute(out mute);
        Marshal.ReleaseComObject(volume);
        return mute;
    }

    public static void SetApplicationVolume(int pid, float level)
    {
        ISimpleAudioVolume volume = GetVolumeObject(pid);
        if (volume == null)
            return;

        Guid guid = Guid.Empty;
        volume.SetMasterVolume(level / 100, ref guid);
        Marshal.ReleaseComObject(volume);
    }

    public static void SetApplicationMute(int pid, bool mute)
    {
        ISimpleAudioVolume volume = GetVolumeObject(pid);
        if (volume == null)
            return;

        Guid guid = Guid.Empty;
        volume.SetMute(mute, ref guid);
        Marshal.ReleaseComObject(volume);
    }

    private static ISimpleAudioVolume GetVolumeObject(int pid)
    {
        // get the speakers (1st render + multimedia) device
        IMMDeviceEnumerator deviceEnumerator = (IMMDeviceEnumerator)(new MMDeviceEnumerator());
        IMMDevice speakers;
        deviceEnumerator.GetDefaultAudioEndpoint(EDataFlow.eRender, ERole.eMultimedia, out speakers);

        // activate the session manager. we need the enumerator
        Guid IID_IAudioSessionManager2 = typeof(IAudioSessionManager2).GUID;
        object o;
        speakers.Activate(ref IID_IAudioSessionManager2, 0, IntPtr.Zero, out o);
        IAudioSessionManager2 mgr = (IAudioSessionManager2)o;

        // enumerate sessions for on this device
        IAudioSessionEnumerator sessionEnumerator;
        mgr.GetSessionEnumerator(out sessionEnumerator);
        int count;
        sessionEnumerator.GetCount(out count);

        // search for an audio session with the required name
        // NOTE: we could also use the process id instead of the app name (with IAudioSessionControl2)
        ISimpleAudioVolume volumeControl = null;
        for (int i = 0; i < count; i++)
        {
            IAudioSessionControl2 ctl;
            sessionEnumerator.GetSession(i, out ctl);
            int cpid;
            ctl.GetProcessId(out cpid);

            if (cpid == pid)
            {
                volumeControl = ctl as ISimpleAudioVolume;
                break;
            }
            Marshal.ReleaseComObject(ctl);
        }
        Marshal.ReleaseComObject(sessionEnumerator);
        Marshal.ReleaseComObject(mgr);
        Marshal.ReleaseComObject(speakers);
        Marshal.ReleaseComObject(deviceEnumerator);
        return volumeControl;
    }
}

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

internal enum EDataFlow
{
    eRender,
    eCapture,
    eAll,
    EDataFlow_enum_count
}

internal enum ERole
{
    eConsole,
    eMultimedia,
    eCommunications,
    ERole_enum_count
}

[Guid("A95664D2-9614-4F35-A746-DE8DB63617E6"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
internal interface IMMDeviceEnumerator
{
    int NotImpl1();

    [PreserveSig]
    int GetDefaultAudioEndpoint(EDataFlow dataFlow, ERole role, out IMMDevice ppDevice);

    // the rest is not implemented
}

[Guid("D666063F-1587-4E43-81F1-B948E807363F"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
internal interface IMMDevice
{
    [PreserveSig]
    int Activate(ref Guid iid, int dwClsCtx, IntPtr pActivationParams, [MarshalAs(UnmanagedType.IUnknown)] out object ppInterface);

    // the rest is not implemented
}

[Guid("77AA99A0-1BD6-484F-8BC7-2C654C9A9B6F"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
internal interface IAudioSessionManager2
{
    int NotImpl1();
    int NotImpl2();

    [PreserveSig]
    int GetSessionEnumerator(out IAudioSessionEnumerator SessionEnum);

    // the rest is not implemented
}

[Guid("E2F5BB11-0570-40CA-ACDD-3AA01277DEE8"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
internal interface IAudioSessionEnumerator
{
    [PreserveSig]
    int GetCount(out int SessionCount);

    [PreserveSig]
    int GetSession(int SessionCount, out IAudioSessionControl2 Session);
}

[Guid("87CE5498-68D6-44E5-9215-6DA47EF883D8"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
internal interface ISimpleAudioVolume
{
    [PreserveSig]
    int SetMasterVolume(float fLevel, ref Guid EventContext);

    [PreserveSig]
    int GetMasterVolume(out float pfLevel);

    [PreserveSig]
    int SetMute(bool bMute, ref Guid EventContext);

    [PreserveSig]
    int GetMute(out bool pbMute);
}

[Guid("bfb7ff88-7239-4fc9-8fa2-07c950be9c6d"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
internal interface IAudioSessionControl2
{
    // IAudioSessionControl
    [PreserveSig]
    int NotImpl0();

    [PreserveSig]
    int GetDisplayName([MarshalAs(UnmanagedType.LPWStr)] out string pRetVal);

    [PreserveSig]
    int SetDisplayName([MarshalAs(UnmanagedType.LPWStr)] string Value, [MarshalAs(UnmanagedType.LPStruct)] Guid EventContext);

    [PreserveSig]
    int GetIconPath([MarshalAs(UnmanagedType.LPWStr)] out string pRetVal);

    [PreserveSig]
    int SetIconPath([MarshalAs(UnmanagedType.LPWStr)] string Value, [MarshalAs(UnmanagedType.LPStruct)] Guid EventContext);

    [PreserveSig]
    int GetGroupingParam(out Guid pRetVal);

    [PreserveSig]
    int SetGroupingParam([MarshalAs(UnmanagedType.LPStruct)] Guid Override, [MarshalAs(UnmanagedType.LPStruct)] Guid EventContext);

    [PreserveSig]
    int NotImpl1();

    [PreserveSig]
    int NotImpl2();

    // IAudioSessionControl2
    [PreserveSig]
    int GetSessionIdentifier([MarshalAs(UnmanagedType.LPWStr)] out string pRetVal);

    [PreserveSig]
    int GetSessionInstanceIdentifier([MarshalAs(UnmanagedType.LPWStr)] out string pRetVal);

    [PreserveSig]
    int GetProcessId(out int pRetVal);

    [PreserveSig]
    int IsSystemSoundsSession();

    [PreserveSig]
    int SetDuckingPreference(bool optOut);
}