I have submitted the following feedback to <www.apple.com/feedback> :
"Subject: iOS 11.3-11.4.1 audio 'bug' (iPhone 6)
Feedback type: bug report
Comments: The latest iOS has a BAD audio bug. (using iOS11.4.1 on an iPhone6; updated from 11.2.6 yesterday). After the update I resumed an audiobook I was playing before and started hearing distortions almost immediately. The issue is consistent regardless of the playback/output settings (through the internal speakers, headphones, and bluetooth). It is triggered whenever the phone screen locks/sleeps: the sound cuts out briefly then skips, distorts and echoes (same for all audio; restart, reinstall, restore do not fix). HUNDREDS of people are talking about this in apple communities (discussions.apple.com/thread/8339248) and apple is risking the loss of loyal customers by not addressing this issue! These phones are (supposed to be) media players for Christ's sake! PLEASE STOP IGNORING THIS! PLEASE!"
I would suggest to anybody that is dealing with this audio 'bug' to please submit feedback to apple and tell them you are having this problem. Surely there is only so much they can turn a blind eye to. This appears to be a problem for a lot of people (there are hundreds and hundreds of posts about it in this thread alone) and it is totally unacceptable that Apple can't even be bothered to feign concern regarding the issue, let alone fix it. These phones are meant to be multimedia devices and an audio bug of this magnitude is not something that they can simply brush aside or ignore and expect the problem to go away--the only way that's going to work is if everybody suffering this issue finally gives up on Apple and buys an android, and for me, at least, that will be a sad day! I love my iPhone, but I listen to my headphones constantly and this 'bug' simply makes audio unlistenable.
Backing up and restoring your iOS will work temporarily (most people report 4 or 5 days of relief), but this will not permanently fix the problem.
Our best bet is to pester Apple until they are forced to take the problem seriously and consider fixing it (our complaints need to assail their screens like an iOS-software-update-reminder). So far they have blatantly ignored every plea for help they have received, but with perseverance on our part and a little luck, perhaps they'll eventually cave.
Meanwhile, turning the automatic screen-lock off and keeping your phone plugged directly into a charger (not an intermediate power source) seems to keep most of the distortions at bay.
Good luck guys!