These suggestions have not helped, and you don't want to hard reset your phone. This is what i think worked for me.
1. Plug in headset when iPhone is locked.
2. Hold down the power button so that the "slide to power off" bar appears.
3. Remove the headphones.
4. Push the "Cancel" button at the bottom of the screen.
5. Go to the phonepad and see if pressing the buttons now produces sound.
(It's possible that I've forgotten something that I did, so the next time my phone locks, I'll try the same method again and update this post. Unless someone else is having the problem and wants to verify.)