I have Audi A4 SE Tecknik 2015 and the bass pops all the time and sound quality is awful through the Music app on iPhone XS Max. Also tested on iPhone 7 and it's the same. I actually took my car back to Audi as I thought the sound system was faulty at first but they found no problems with the Audi sound system when doing a wide range frequency test.
I have reported the bug to Apple via Bug Reporter. They got back to me and asked for iPhone sysdiagonse logs so I have done that for them. If enough people do the same and complain they might release a fix.
runexnilsen is correct. I had the problem almost immediately upon downloading iOS 12, playing music files -- not streaming -- in my car on my iPhone 7. As others have noted, it isn't that the music was distorted all the time, but sometimes at different volumes and combinations of treble and bass. I didn't have the problem at al playing the same files off my MacBook. I downgraded my iPhone back to iOS 11.4 (which I gather can no longer be done) and the problem disappeared as instantly as it had appeared. It's certainly some combination of iOS 12 and Bluetooth that causes the problem. If anyone should find the problem improves with a subsequent update of 12, I'd love to hear about it, as I imagine others would as well. Thanks.
Following up on the above: I had a chat with an Apple Tech Adviser last night who claimed that the new iOS 12.0.1 may resolve Bluetooth issues beyond just the stated "connectivity" problem. If anyone in this conversation finds that their music playback is improved by the new update, can you please post it? Thanks.
I had an IPhone 8 Plus work fine for a year with various Bluetooth devices:
2 x Bose Headsets
C Class W205 Burmeister Sound System
Upon upgrading to IOS 12 I noticed this issue straight away in my car, trebel sounds fading in and out every 20 seconds or so, really irritating...
Checked the internet and noticed this appears to be an IOS issue.
It does this on ITunes music also...
I Reported to apple several times, after a full hard reset via iTunes and upgrading to IOS 12.0.1 today, unpairing and re-pairing Bluetooth connections with my car, this issue still remains.. :-(
The strange thing is, I only notice this in my car, so it must be related to a Bluetooth profile it uses or something.
Please all escalate this to apple.
Apple intialy told me my Bluetooth device was wearing out and to get a new device or contact manufacturer :-) I then said people are rolling back to IOS 11 and all is fixed, they were then offering to call me tomorrow and run some diagnostics on my phone.
I will keep you all updated on how this works out.
Thanks for your post, Dan. As I indicated in my previous two posts, I'm having a problem very similar to yours, which presented itself immediately in my car (but nowhere else) with the download of 12 and then immediately went away when I reverted to 11.4. Very sorry to hear that apparently the new upgrade of 12.0.1 isn't resolving things.
Thanks for creating this ticket. Unfortunately I've tried to get onto this page to no avail; I keep getting messages about too many "redirects," no matter how many different ways I try. I do notice, however, other community threads similar to ours raising the same issue.