Volume Manipulation Classes

Many times my applications required audio volume manipulation.
To make the volume-enabled application development easier I decided to
create a few C++ classes that would allow me to easily regulate and track
the changes of such volume controls as Output Master Volume, WaveOut Volume and
Input (WaveIn) Volume.
Here I provide such classes that share a common interface (defined in IVolume.h):

bool	IsAvailable() - Says whether the volume controling is possible
void	Enable() - Enables the line of the volume control
void	Disable() - Disables the line of the volume control
DWORD	GetVolumeMetric() - Retrieves the granularity of volume
DWORD	GetMinimalVolume() - Retrieves the minimal volume that can be set
DWORD	GetMaximalVolume() - Retrieves the maximal volume that can be set
DWORD	GetCurrentVolume() - Retrieve the current volume
void	SetCurrentVolume( DWORD dwValue ) - Set the volume

And the last function allows to register a user-implemented callback that
will be called as a notification of volume changes:
void	RegisterNotificationSink( PONMICVOULUMECHANGE, DWORD )

This interface is implemented by CVolumeOutMaster (VolumeOutMaster.h/cpp),
CVolumeOutWave (VolumeOutWave.h/cpp) and CVolumeInXXX (VolumeInXXX.h/cpp) classes.
The usage of the classes is very simple:
In your StdAfx.h include "mmSystem.h" and make sure you link to the "winmm.lib"
( #pragma comment(lib, "winmm.lib") ).
Then, if you are going to use Output Mater volume control, include "VolumeOutMaster.h",
say, to the StdAfx.h
The IVolume.h, VolumeInXXX.h, VolumeInXXX.cpp are to be inserted as your project files.
...
void CALLBACK MasterVolumeChanged( DWORD dwCurrentVolume, DWORD dwUserValue );
...
// Volume control Initialization
IVolume* pMasterVolume = (IVolume*)new CVolumeOutMaster();
if ( !pMasterVolume || !pMasterVolume->IsAvailable() )
{
 	// handle error
}
pMasterVolume->Enable();
pMasterVolume->RegisterNotificationSink( MasterVolumeChanged, dwAnyUserValue );
...
pMasterVolume->SetCurrentVolume( dwVolumeToSet );
...
DWORD dwCurrentVolume = pMasterVolume->SetCurrentVolume();
...
void CALLBACK MasterVolumeChanged( DWORD dwCurrentVolume, DWORD dwUserValue )
{
	// handle the volume change
}
...

Very simple, isn't it?
Yet, the CVolumeInXXX class requires more explanation. In order to manipulate
the Input volume, the source line index is to be passed to the constructor.
Confused? Please, be not. CVolumeInXXX class provides a static function to
enumerate those lines:

bool	EnumerateInputLines( PINPUTLINEPROC, DWORD dwUserValue );

This allows you to manipulate the volume of any WaveIn-based lines.
Say, you want to manipulate the microphone volume:
...
bool CALLBACK EnumInputLineProc( UINT uLineIndex, MIXERLINE* pLineInfo, DWORD dwUserValue );
...
// Initialization
UINT uMicrophoneLineIndex = (UINT)-1;
if ( !CVolumeInXXX::EnumerateInputLines( EnumInputLineProc, (DWORD)&uMicrophoneLineIndex ) )
{
	// handle error
}
if ( uMicrophoneLineIndex == (UINT)-1 )
{
	// Error: mic volume'ing is not available.
}
IVolume* pMicrophoneVolume = (IVolume*)new CVolumeInXXX( uMicrophoneLineIndex );
if ( !pMicrophoneVolume || !pMicrophoneVolume->IsAvailable() )
{
 	// handle error
}
// Go on and use pMicrophoneVolume to manipulate the volume
...
bool CALLBACK EnumInputLineProc( UINT uLineIndex, MIXERLINE* pLineInfo, DWORD dwUserValue )
{
	if ( pLineInfo->dwComponentType == MIXERLINE_COMPONENTTYPE_SRC_MICROPHONE )
	{
		*((UINT*)dwUserValue) = uLineIndex;
		return false;
	}
	return true;
}
...

Be aware, that for performance reasons it is better to have a single instance
of a given class per application. So don't rush to create lots of CVolumeInXXX
objects, better share the only one through your code.

Conclusion:
The proposed classes do not encapsulate all the abilities exposed by the mixers.
However, working with a mixer just to add a pretty simple functionality is quite boring.
That's why, as I think, the proposed classes might be of some help to you.