1) Has anyone reproduced this on iOS 11 on an older device? That way we can figure out if it is related to iOS or to the combination of iOS+iPhone 8/Plus.
2) Seems like everybody with iPhone 8 or 8 Plus is affected. Are there any reports of people who is not affected?
To summarize: this manifests on calls (voice or wi-fi), only by using the earpiece, with every carrier on both iPhone 8 and iPhone 8 Plus. It does not happen while listening to music, voicemail, etc through the earpiece. Triggering the speaker can turn this effect on and off. In never manifests through the speaker or headphones. Hard reset and firmware restore do not make the issue go away.
Worst case scenario: if it is hardware (which I think is unlikely), there is a consistent way to "fix" it, which means the developers can introduce a software "workaround" to patch the problem, even if the root cause is in hardware.