I had the same problem. The first time, after 900 miles, there was no sound but the screen showed the fm station and then the navag. info, but no sound. I came back from Starbucks, and unlocked the car and there was sound. The next weekend when I drove the car to Starbucks at 35 mph, the same problem, no sound. I came back to the car and the spoiler was up and the sound was fine. The dealer ran a diagnostic test and there was a fault with the amplifier and it was replaced and now the sound is okay. Will watch for the spoiler function.
Otherwise, the 997S is perfect and the most fun and the best car I have owned since my first 911T in 1969 and the subsequent 30 plus 911s I have enjoyed over the years. My Cayenne turbo was the ultimate and best SUV.