I'm having the same problem and currently chatting online to technical support - simply cannot hear person on the other end of the call, but switching to speakerphone or putting earphones in works fine. It is definitely a problem with the update, the amount of people on here with problems after the update is clearly a testament to that. I have power the device off and on, done a hard reset, and even changed out the earpiece speaker for another one....and still it persist. What really boils my blood though is how Apple deals with these instances - just be honest and come out publicly....admit to the fact there is a glitch somewhere and you are doing everything to fix it. We all make mistakes - we can relate to that.
What I seriously dislike is what I'm currently experiencing with technical support. You explain the issue what and the first quesiton they ask is "has the device been dropped or had any liquid damage?" - I have to smile about it. They know there is a problem. I dare say even this thread has been brought to their attention. I've been very forthright (but polite) in telling them this is a problem with the update, but they insist it may be a hardware issue that needs repair. I told them I fix phones and swapped out the earpiece speaker last night (phone is outside of one year warranty) and the defences shoot up telling me now the device has been modified it may be beyond economical repair. I asked them to just confirm that this was a software problem, to which I was told they have had no reported instances of this problem. That was when my blood started to boil - do they really need to lie and cheat like this? Are the billions not enough? After pushing them on this and explaining plenty of other people were in the same situation I was told that Apple hadn't issued any company wide memos regarding this issue - I guess they just hope most people are idiots and will continue to line their pockets with replacement device fees.
I've now been transferred to a "senior advisor" who now admits there is quite a bit of "chatter" on the forums. I've been asked to restore the phone as new to see whether it is a hardware or software problem. Surely restoring the phone to the same software that created the problem will solve nothing. Guess I'll have to try it out and update on here once it's done....