It kind of sounds like Siri might not be activated. Go to Settings - General - Siri and confirm it's turned on. If it is, you might also make sure "raise to speak" is turned on, then go to your home screen and raise your phone to your ear. Does the Siri audible prompt sound?
If nothing else works, I'd do a current sync, then go to Settings - General - Reset and do a full reset. If that doesn't work, try a Restore. Here's a link for how to do that: http://www.youtube.com/watch?v=Jzyy4puapig You won't lose any data or apps, but you will have to reinput some odds and ends like your WiFi password. Good luck.