2019 Subaru Forrester Touring w H/K 9 speaker surround system.
2018 GMC Terrain SLT w Bose 7 speaker surround system.
On both vehicles, we have had the same issue with both an iPhone 6+ and iPhone 7+ on iOS 12.3.1 with CarPlay.
Once I disabled CarPlay altogether the audio issue went away completely, and remain so.
Meanwhile, if I use my iPhone as an USB iPod through Media, no issues, crystal clear audio all the way; over BT, no issue either.
To me, it sounds like a vinyl record skipping periodically.
I called Subaru, they said it’s an “Apple issue” and an update will be coming soon. I have the latest head unit firmware for the car, and I’ve tried five different cables now, three of which are Apple-branded.
For now, I’m happy just using it as an iPod as a workaround but would love to test it with Android Auto to see if there are any differences in the sound quality.