I wish I had some better news. Talked to my SA. He checked the (allegedly identical) unit in his Macan. It operated (or, more aptly, failed to operate) in the same fashion. The conclusion is that there is nothing wrong, no incorrect settings. This is a feature. We can only adjust the volume during navigation by coupling it to the radio. I'm sure this would be bad enough in a Cayman, but in a Boxster with the top down, you would need to have music blaring to hear anything.
I may have said this before, and if I did, I don't remember what it was about - but this is absolutely incomprehensible to me. This is ridiculous; stupid. Insultingly so.
I read with interest that JD Power has come out with some ratings on initial quality, and I believe Porsche was fourth overall, second in the luxury segment, behind some Asian manufacturers. But what really caught my eye was that infotainment systems were the focal point of complaints nowadays.
It's the market, I know. But how the market can demand this kind of crap in their cars, where it doesn't really belong, and then still clamor for it when it doesn't work, is another thing that I'll never be able to understand.
I hope that I don't have the complete story yet. I'd love it for people to post about how their systems work, and see if we can determine any common denominators (country, trim level, date of manufacture, etc.) among any who are able to adjust to volume of their navigation systems.
And let me guess. If I figure out how to involve my phone, with Carplay and Connect+, in navigation - I'll bet we don't get voice prompts at all, or something equally moronic. And the manufacturers will just keep jingling the car keys in front of these infants.