' Look for the ComboBox drop down.
Public Function NewWindowProc(ByVal hWnd As Long, ByVal msg _
    As Long, ByVal wParam As Long, ByVal lParam As Long) As _
    Long
Const WM_NCDESTROY = &H82

    ' If we're being destroyed,
    ' restore the original WindowProc.
    If msg = WM_NCDESTROY Then
        SetWindowLong _
            hWnd, GWL_WNDPROC, _
            OldWindowProc
    End If

    If msg = WM_COMMAND And _
        ComboHwnd = (lParam And &HFFFF) And _
        (wParam \ &H10000) = CBN_DROPDOWN _
    Then
        ' The ComboBox is being dropped down.
        Beep
    End If

    NewWindowProc = CallWindowProc( _
        OldWindowProc, hWnd, msg, wParam, _
        lParam)
End Function