Looks like I'm the latest member of this club, because I'm now struggling to have my mic pick up my voice when either talking to Cortana, or attempting to record in portrait (like several of you have pointed out, landscape mode works perfectly).
I've tried inserting and removing my headphone jack, because I'd definitely qualify as a frequent user of headphones, but that isn't working for me. Because I bought my Lumia off-contract, it's not under warranty, so I'm left with either taking it apart myself (something I'm not at all eager to consider), or hoping a software fix gets to the bottom of this. I'm on Denim, and this is frustrating.