iOS 11.3.1, an iOS update released to fix unresponsive iPhone 8 handsets repaired with third-party displays, is itself buggy.
I've come across reports of two issues.
The first is an audio skipping issue (which also seemed to be present in iOS 11.3, but iOS 11.3.1 seemed to bring this problem to people who didn't have it previously). This problem seems to vary from simple audio skipping and stuttering, to crackles, pops, and other random distortions.
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This problem seems to affect both Apple's own audio apps -- even on phone and FaceTime calls -- as well as third party apps.
Another issue that's been reported to me by several iPhone users is that the iOS 11.3.1 brings to the iPhone 6s the problem it was meant to fix on if the iPhone 8 -- display unresponsiveness on devices that were fixed using third-party parts.
If you own an iPhone that's had the display replaced with a third-party part and you've not yet upgraded to iOS 11.3.1 then it might be a good idea to give this update a miss and give Apple some time to investigate this issue.