The MCIWNDM_SETVOLUME message sets the volume level of an MCI device. You can send this message explicitly or by using the MCIWndSetVolume macro.
MCIWNDM_SETVOLUME 
wParam = 0; 
lParam = (LPARAM) (UINT) iVol; 
 
Returns zero if successful or an error otherwise.
  Windows NT: Requires version 3.1 or later.
  Windows: Requires Windows 95 or later.
  Windows CE: Unsupported.
  Header: Declared in vfw.h.
MCIWnd Window Class Overview, MCIWnd Messages, MCIWndSetVolume