I have the same issue with the 12" UConnect and HK speakers. When I am using CarPlay to listen to music while using the Waze or Google Maps app for guidance, the music will be playing loud on all speakers and then suddenly the driver's speakers will stop playing the music and play an announcement for an upcoming turn, etc. The volume for the turn directions is very low. But to make it worse, the other speakers are still blaring the music.
I had a Bluetooth issue on my Harley where I had to turn the volume on the bike up really loud to hear music on my phone. Then, if I forgot to turn it back down, I would switch to something else like the radio and the volume would be ear shattering. This was resolved buy deleting the paired phone, turning the volume up on my phone, and then re-pairing it with the bike. CarPlay is using a wired connection. And the volume is fine for the music. Any suggestions?