Events In Excel VBA

Hi,

I am trying to create a macro that, every time a celll value changes by a tenth of a point (either up or down), then excel speaks the new value in the cell.

The code below is what I have implemented:
1. Open the Visual Basic Edit (Tools>Macro>Visual Basic Editor)
2. In the left window, right click Sheet1 and select View Code.
3. At the top of the Book1 - Sheet1 Code dialog box, there are two dropdowns. From the left dropdown select Worksheet.
4. Copy and Paste the following code:
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$A$1" Then
Target.Speak
End If
End Sub

There are two problems, in order of importance, with the above code:
1. If cell "A1"" contains a formula, it does not recognise the fact that a the cell value has changed and thus does not speak the new value. (This is vital as the cell I want excel to speak is updated automatically from an external application running simulatneously)
2. It speaks when the value changes by less than a tenth of a point.

Any help would be greatly appreciated, including a relevant text book recommendation.

Many thanks!

James