I have a 2017 S75 with standard audio and no problems with Nav audio.
Interesting. So you mean that you can hear the nav voice if driving at motorway speed above 70mph and the music fade down on the left hand side speakers? If I have the nav volume to 11 and run some music at 7+ volume and there is a nav voice guidance the music doesn't fade down enough and it is impossible to hear what the nav voice say.
Do you have the new navigation? Because that could maybe solve the issue. I'm still on the old Navigon system here in Europe.
If I don't have any music on, it is no problem to hear the nav voice. And like I have written many times, in cars with premium sound it works like it should. I've tested this with at least 6 different teslas. All with standard audio got the problem and all with upgraded sound works fine.