After over a year of flawless use, my 930 has just developed the silent microphone problem.
However, there is a bit of a twist. If I record video in portrait there is no recorded sound - but in landscape there is!
Also, the speaker and microphone setup appears to be in reverse. i.e. If I use Skype without the speakerphone, the microphone is very sensitive and will pick up my voice from across the room. If I enable the speaker then the microphone is muted.
Interestingly "Hey Cortana" now seems to work from distance whereas up until recently it didn't.
It sounds like something has been updated to make the mic more sensitive in normal use but at the expense of being used with the speakerphone.
I can't see how this is hardware related but it looks like the only solution is to send it back anyway!