I've been having the same issue and I have no idea why. Performed a factory reset. Tried running the phone in safe mode. It seems like it works briefly after a restart but then cuts out after a few minutes. I never had any issues like this before the update.
Problem for me goes as follows.
Receive phone call
Try to answer
Either the phone answers the call and there is no sound (mic or speaker) but otherwise acts normal or the phone refuses to answer the call and the "swipe up to answer" UI keeps reappearing. When the later happens the phone crashes and goes through a restart that takes longer than a normal one.
Make a phone call
No sound (mic or speaker)
The issues with the microphone seems to affect other parts of the system as well, such as ok Google and voice typing.
Speaker seems to work fine other wise
Putting the phone in safe mode seems to delay the problem sometimes.
Same thing started happening for me yesterday!
I was sending a voice message on Whatsapp.
The msg was sent but no audio was recorder.
I received a call but there was no audio (both in & out)
I tried making a call and the same happened.
The "Phone" app would even hang after hangup and would crash. I get a white screen with the G logo and a loading bar beneath it, then it would start.
Starting the phone app is also sluggish.
If I make a call (and get no audio) I try to switch to loudspeaker and I could hear the other party but for a couple seconds then it's muted, then comes back for a couple seconds, and keeps doing it intermittently.
Sound from music app and videos works fine.
SOMETIMES after a reboot phone audio&mic would return but temporarely.
SOMETIMES when I try rebooting and it doesn't work, I try safe mode and it work (not all the time though)
SOMETIMES switching from LTE to WCDMA only fixes it temporarly.
This is really unpredictable.
I reverted back to a 7.1.2 stock. At first the issue persisted then it was fixed for a couple hours.
I updated back to O and audio worked for some time but after a few minutes, the issue returned.
Now I am on a clean stock install of O. without any apps installed. The audio comes back for a few minutes, then it's gone for hours.
Hopefully this thread will grab the attention of someone who might be able to help