I think you would need to deduce the sheet and cell from the link address and check the value in Excel.
Something like this, treat with care as it is untested and I cannot see you slide!
Sub read_XL()
Dim oshp As Shape
Dim XLaddress As String
Dim rayAddress() As String
Dim oxl As Object
Dim owb As Object
Dim iRow As Integer
Dim ipos As Integer
Dim iCol As Integer
Dim osheet As Object
Set oshp = SlideShowWindows(1).View.Slide.Shapes("P11")
XLaddress = oshp.LinkFormat.SourceFullName
rayAddress = Split(XLaddress, "!")
ipos = InStr(rayAddress(2), "C")
iRow = CInt(Mid(rayAddress(2), 2, ipos - 2))
iCol = CInt(Mid(rayAddress(2), ipos + 1))
Set oxl = CreateObject(Class:="Excel.Application")
Set owb = oxl.Workbooks.Open(rayAddress(0)) ' assumes file is not open
'' you could also check all open files to find it
MsgBox owb.Sheets(1).Cells(iRow, iCol).Value
oxl.Quit
End Sub