prova così ...
codice:
 For parla As Integer = 0 To DataGridView1.RowCount - 1 Step 1
            Synt.SpeakAsync(DataGridView1.Item(5, parla).Value)
            Dim indice As Integer = parla
            Me.DataGridView1.Item(5, indice).Selected = True
Application.DoEvents 
            DataGridView1.CurrentRow.DataGridView.CurrentRow.DefaultCellStyle.SelectionBackColor = Color.Green
            DataGridView1.CurrentRow.DataGridView.CurrentRow.DefaultCellStyle.SelectionForeColor = Color.Red
Application.DoEvents 
        Next
Facci sapere....
Ciao