OK, I found a way by comparing the OLD and NEW text.
Not sure it is the best way, but it works.
I am still open to know the correct/proper or the best way.
B4X:
Sub txtNewReading_TextChanged (Old As String, New As String)
If New.Length > Old.Length Then
Old = Old & "!"
For i = 0 To Old.Length-1
If Old.SubString2(i,i+1) <> New.SubString2(i,i+1) Then
Dim KeyPressed As String = New.SubString2(i,i+1)
MP.Load(File.DirAssets, KeyPressed & ".mp3"): MP.Play
Exit
End If
Next
Else
MP.Load(File.DirAssets, "Delete.mp3"): MP.Play
End If
End Sub