type
       TForm1 = class(TForm)
       private
       {Private declarations}
       procedure WMSysCommand(var Msg: TWMSysCommand); 
       message WM_SYSCOMMAND;
       public
       {Public declarations}
end;

var
       Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.WMSysCommand;
begin
       if (Msg.CmdType = SC_MINIMIZE) or (Msg.CmdType = SC_MAXIMIZE) then
       MessageBeep(0)
       else
       inherited;
end;