How can I get the audio prompts working again while using fenix 5 during interval training workouts and fixed zone runs?

Hi, 

if I do interval training and I have to run and rest cyclically I would like to hear some audio prompts telling me when I have to run and when I have to rest. 

Similarly, I have a few workouts where I have to run in zone 2 for a long time, I would like to hear some audio prompts telling me if I reach zone 3 so I can modulate my effort. 

Such alerts seem not presents or not working. Is this normal? how do I turn on such a feature? 

please note that If I turn on the normal audio prompts related to laps and heart rate zone they work properly ((Garmin devices -> device -> activity options -> audio prompts is configured) but I do not receive any audio alerts about the intervals and when I reach other zones.