The MCIWNDM_GETVOLUME message retrieves the current volume setting of an MCI device. You can send this message explicitly or by using the MCIWndGetVolume macro.
MCIWNDM_GETVOLUME 
wParam = 0; 
lParam = 0; 
 Returns the current volume setting. The default value is 1000. Higher values indicate louder volumes, lower values indicate quieter volumes.
  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, MCIWndGetVolume