The app will do this for you automatically – choose ‘Dim between turns’ in App Settings, and it’ll dim the screen to black after each turn, and wake it up in time for the next one.

And if I want both the dimming and the audio, is that maybe possible ?

Oh my bad, the answer is "yes" it's in the settings :)

I have been our testing today (walking because of snow)

My screen does not dim at any time :(

Some of the behaviour of the app is different at slow speeds – it’s not really intended for walking navigation! Try it when the snow clears and let me know how you get on.