Hi all, this is exactly the issue I'm facing! I'm fuming!
My phone was perfectly fine.
I updated to 11.4.1.
My phone instantly lost it's MIC and speaker during phone calls.
The speaker icon is greyed out during an attempted phone call.
SIRI no longer works, because obviously the MIC can no longer hear me.
Voice memos no longer work as the record button can't be pressed (Again, MIC isn't working).
Went through resets, restores. Then all again with apple employee via twitter.
Apple then basically told me I'll have to pay for it to be repaired!
What are we supposed to do when Apple basically tell me that I have to pay for the damage they have done to be repaired?! 
I have posted this on the Martin Lewis forum seeking views etc, I hope I'm OK to post the link here.
https://forums.moneysavingexpert.com/showthread.php?t=5869622
I also posted a tweet seeking others who have suffered this issue:
https://twitter.com/TekNeil/status/1019350213419511814
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EDIT: OK, I just sent feedback to Apple as suggested by the guy I quoted. Here's what I sent in case anybody wants to use it as a template.