Voice coaching is only available in specific training modes, not in general across the watch. Make sure you have the Voice on by pressing down and selecting Profile>Voice and your language. You will get the following prompts:
Watch ready, Demo mode, Bluetooth not working when swimming.
Training Partner prompts:
Zone: in zone, above zone, below zone.
Goal: at 50%, 90% 100% and 110% of goal.
Race: ahead, behind, lost, won.
So unless you are in Zone, Goal or Race you will not get any prompts. In lap mode it will buzz to inform you of a completed lap and show the lap time on the watch but there are no audio prompts for it.
You only get prompts when you leave or behind the required pace for the race. So if you get ahead and stay ahead it will not alert you again (you would need to look at the screen to see the arrow telling you to slow down). It does not constantly alert you and seems to miss a lot of alerts.