I also got this issue. And it is interesting that this issue comes right after update is installed(mine broke when I have installed dev.preview 8.1). I was enjoying the update, cortana worked well, and then suddenly probem occured. Now my updates:
-I have Beats earphones with mic on cord, and when I plug it into my 925, push record sound and dictate something, nothing is recorded. But when I do the same but do not dictate and just PRESS the buttons that are located on cord(pick-up-phone-call button, volume up and down) I can hear on playback my pressings on buttons. I mean I hear clicks, fuzzy and yet clicks. To be honest, I dont understand what that means, but I guess it suggests that mic is not hardware broken.
-But above post also suggests that our issue can be solved hardware wise. I think I will also spend another 5-6 bucks and order that second mic. Replace it and do not wait until MS will push update that will fix our problem.
P.S: disassemble of our L925 is not that difficult according to videos.