Poor CarPlay sound quality

@Scott I completely empathize with your situation as I have had struggles with CarPlay compatibility between two (2) vehicles.

With iOS 13.2 and the the latest/greatest head unit (HU) firmware for my vehicle (specs below), CarPlay has gotten better sans two (2) very specific struggles/bugs I continue to have...

• Audio Quality
  • Overall audio remains flat, narrow, and at-times (as you stated) shrill.. In lay terms, the audio or harmonic "range" is definitely more narrow than when I play music over BT, or over USB with CarPlay disabled (e.g., iPod Mode).
    • The Bass is flat. --there's no other way to say it; it's as though the sub is disconnected or non-existent.
    • The Treble is too narrow. --Tweeters, anyone?
    • The Mid Range is overly emphasized. --This is fine for Podcasts if that was all I listened to.
• GPS Navigation
  • iOS GPS applications, namely Apple Maps, give mixed location monitoring performance compared to the integrated Infotainment nav system.
    • For example, randomly, my location on Maps will display 20-30' outside the passenger-side of the vehicle.
    • Direction takes a long time to re-orient, especially when in a parking lot; it will show me driving in the opposite direction I am going.
    • Likely, this could be fixed if CarPlay and/or the Auto Maker shared GPS coordinates (i.e., roof antenna), vehicle motion/speed, etc., but it remains inconsistent between vehicles. --My last vehicle did the same thing so it's a gamble which HU interfaces GPS, speed, etc., with CarPlay.

The best advice I can give you is keep your HU up to date and use a quality Lightening cable with your iPhone. Meanwhile, I recommend letting Apple know of your struggles: apple.com/feedback

I've sent several feedbacks to both Apple and my automaker with relation to these glitches. Meanwhile, I continue to use my iPhone via USB in "iPod mode" with full audio range and long-press the voice button to occasionally access Siri for music selections.

'19 Subaru Forester Touring (H/K 8 Speaker System)

• Anecdotally, since the iOS 13 update, the Media selection system on the Infotainment HU no longer works 100% with the iPhone. For example, it shows duplicates of tracks and podcasts, and will not activate songs, Podcasts, audio books, etc. --this all worked perfectly in iOS 12 so the update to iOS 13 clearly changed something.

iPhone 7 Plus (iOS 13.2)

You are absolutely correct. I hear crackling and distortion, particularly with vocals. The music is lifeless, with no dynamic range or punch. I have ruled out my phone output hub, the USB cable, the USB hub and the car speakers in my new Mercedes. It's definitely the new Airplay software upgrade. It occurred first on the very first day of the update. Airplay has changed the line signal output characteristics. I strongly suspect that Apple is using dynamic range compression and/or downsampling and driving the line output level higher, above the recorded "red lines" in order to go louder-- always louder. Many listeners won't notice but to me, my music is now unlistenable in the car which makes me sad. Steve Jobs was an audiophile and he would be downright horrified. I called Apple but they reacted as if I were crazy. Hopefully somebody is checking these threads and they can add this to the list of OS13 bugs to be fixed.