I am having this exact same issue. I'm using an unlocked iPhone 5 32GB (A1429 - MD300B/A) bought directly in my local Apple Store in Reading, UK.
I went in there today to show that Siri has gone deaf, but works over the headphone cable mic, as well as the Speakerphone not working either. The advice I was given was:
- start iTunes on my PC and do a backup to the PC, and chose Encrypt iPhone backup
- then click on the Restore iPhone option to download iOS 7.0 again and install it over USB to the iPhone
- once that is done and the iPhone reboots, disconnect the Lightning cable and choose the set up the device as a new iPhone (don't restore any backup)
- don't sign up for iCloud
- test Siri to see if its still not working
- plug the sync cable back in, and then restore your backup to get your apps and settings back
- if that still doesn't work, then raise this as an issue on apple.com (the Apple Store staff have no way of logging issues found by customers, if they haven't got AppleCare or haven't booked a Genius Bar appointment, apparently. What cack is this?)
This is quite a lot of work, but I'm getting sick of the various things that are wrong with iOS 7 (the battery life, the crashes) so I gave it a try.
It made no difference whatsoever, Siri was still broken. Worse, the iTunes backup was completely ineffective, after doing a Restore Backup, it had restored nothing, no apps, setting, anything. Yet another thing that is broken. I was in a panic that I'd lost everything. Thankfully I had done an iCloud backup so I could get my stuff back, into Settings > General > Reset and then wait while it was sucked down from Apples datacentres.
How exactly do you raise this as an issue with Apple on this website? I've only found the Feedback page.