function OpenCD(Drive: Char): Boolean;
var
  Res: MciError;
  OpenParm: TMCI_Open_Parms;
  Flags: DWORD;
  S: string;
  DeviceID: Word;
begin
  Result := False;
  S := Drive + ':';
  Flags := MCI_OPEN_TYPE or MCI_OPEN_ELEMENT;
  with OpenParm do
  begin
    dwCallback := 0;
    lpstrDeviceType := 'CDAudio';
    lpstrElementName := PChar(S);
  end;
  Res := mciSendCommand(0, MCI_OPEN, Flags, Longint(@OpenParm));
  if Res <> 0 then
    Exit;
  DeviceID := OpenParm.wDeviceID;
  try
    Res := mciSendCommand(DeviceID, MCI_SET, MCI_SET_DOOR_OPEN, 0);
    if Res = 0 then
      Exit;
    Result := True;
  finally
    mciSendCommand(DeviceID, MCI_CLOSE, Flags, Longint(@OpenParm));
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  // Abrindo o drive de DVD
  OpenCD('G');
end;