看看这个函数。
MCIERROR mciSendCommand(MCIDEVICEID wDeviceID, MCI_SETAUDIO,
DWORD dwFlags, (DWORD) (LPMCI_GENERIC_PARMS) lpSetAudio);
结合使用MCI_DGV_SETAUDIO_ITEM，控制声道。
MCI_DGV_SETAUDIO_SOURCE
A constant specifying the source of audio input is included in the dwValue member of the structure identified by lpSetAudio. The following constants are defined for the audio input sources:
MCI_DGV_SETAUDIO_SOURCE_AVERAGE
The average of the left and right audio channels.
MCI_DGV_SETAUDIO_SOURCE_LEFT
Left audio channel.
MCI_DGV_SETAUDIO_SOURCE_RIGHT
Right audio channel.
MCI_DGV_SETAUDIO_SOURCE_STEREO
Stereo.
其实我也没有做过，不知道能否实现。不过估计只能用这个办法。
