You could use a macro:

Option Explicit
Sub auto_open()
   Dim myStr As String
   
   Select Case Time
       Case Is <= TimeSerial(12, 0, 0)
           myStr = "Good Morning"
       Case Is <= TimeSerial(18, 0, 0)
           myStr = "Good Afternoon"
       Case Else
           myStr = "Good Evening"
   End Select
   
   With Application
       .Speech.Speak Text:=myStr & " " & .UserName
   End With
   
End Sub

ps...

If you're new to macros, you may want to read David McRitchie's intro at:
http://www.mvps.org/dmcritchie/excel/getstarted.htm