On the first retrofit we did, we got that exact same problem. However, that was caused by not having the correct wires at the head unit end. It was pin 5 and 11 for the sound for the navigation unit. However, we had reversed the two cables and we noticed that the radio would "dim" when the voice should come out.
Correcting the positioning at the radio end solved that problem. It is therefore possible that there was just a lose connection at the radio end for this MKIV unit. In other words, I have never seen a sounds failure with this unit.