Since you put C# as a tag, here is a small C# console app that gets it. It's based on the GetMasterVolumeLevelScalar method (Vista or higher).

The GetMasterVolumeLevelScalar method gets the master volume level of the audio stream that enters or leaves the audio endpoint device. The volume level is expressed as a normalized, audio-tapered value in the range from 0.0 to 1.0.

  class Program
  {
      static void Main(string[] args)
      {
          Console.WriteLine(VolumeUtilities.GetMasterVolume());
      }
  }


  public static class VolumeUtilities
  {
      public static float GetMasterVolume()
      {
          // get the speakers (1st render + multimedia) device
          IMMDeviceEnumerator deviceEnumerator = (IMMDeviceEnumerator)(new MMDeviceEnumerator());
          IMMDevice speakers;
          const int eRender = 0;
          const int eMultimedia = 1;
          deviceEnumerator.GetDefaultAudioEndpoint(eRender, eMultimedia, out speakers);

          object o;
          speakers.Activate(typeof(IAudioEndpointVolume).GUID, 0, IntPtr.Zero, out o);
          IAudioEndpointVolume aepv = (IAudioEndpointVolume)o;
          float volume = aepv.GetMasterVolumeLevelScalar();
          Marshal.ReleaseComObject(aepv);
          Marshal.ReleaseComObject(speakers);
          Marshal.ReleaseComObject(deviceEnumerator);
          return volume;
      }

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

      [Guid("5CDF2C82-841E-4546-9722-0CF74078229A"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
      private interface IAudioEndpointVolume
      {
          void _VtblGap1_6();
          float GetMasterVolumeLevelScalar();
      }

      [Guid("A95664D2-9614-4F35-A746-DE8DB63617E6"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
      private interface IMMDeviceEnumerator
      {
          void _VtblGap1_1();

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

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