No sound in apps in iphone 3g?

My iphone was working fine just 2 hours back but now it is not. There's no sound in apps and no lock sound, no key tone. However videos and mp3s are working fine Also no ringtone when call comes. What to do? Please give a solid cure

Make sure your phone is on silent, and make sure the volume is turned up.

If that doesn't work, try turning the phone off/on. If not, you could try restoring the phone as new.