The MCIWNDM_GETDEVICE message retrieves the name of the currently open MCI device. You can send this message explicitly or by using the MCIWndGetDevice macro.
MCIWNDM_GETDEVICE 
wParam = (WPARAM) (UINT) len; 
lParam = (LPARAM) (LPVOID) lp; 
 Returns zero if successful or a nonzero value otherwise.
If the null-terminated string containing the device name is longer than the buffer, MCIWnd truncates it.
  Windows NT: Requires version 3.1 or later.
  Windows: Requires Windows 95 or later.
  Windows CE: Unsupported.
  Header: Declared in vfw.h.
  Unicode: Defined as Unicode and ANSI messages.
MCIWnd Window Class Overview, MCIWnd Messages, MCIWndGetDevice