If that doesn't solve your issue it's always worth actually using the search function to solve this issue, or any other issue you have in the future rather than opening a new thread. If you have checked that you haven't done anything to the side switches as mentioned in the link you've already been given then we need to know whether you have a Bluetooth keyboard. Sometimes this can cause people to lose sound as they've accidentally triggered a key in the keyboard. Good luck!
Proud owner of a 32GB wifi iPad2 iOS5.