I got an iPhone 5 as soon as it came out in September 2012. Three months later, in December, the speaker stopped working. The Genius Bar replaced the speaker but it still didn't work, so they replaced the phone for me under warranty.
Yesterday (October 2013) this phone's speaker stopped working as well. It suddenly went silent in the middle of a phonecall; I wasn't even touching the phone. Nothing could make it make any sound - I tried the mute switch and the volume buttons, I tried turning Bluetooth on and off, I tried cleaning out the headphones jack, I tried resetting all settings on the device. The Genius Bar replaced the speaker and agan it didn't work, but this time I was about 30 days past warranty, so all they could do was to offer me a replacement for $280.
I see lots of other reports of this. I wonder why the iPhone 5 breaks this way so often.