It sounds like you have accidentally turned on mute. This is probably because you had side switch on your iPad on a different setting on IOS4, and you have now turned it on differently on IOS5.
Check in Settings to see what you have "Use Side Switch to:" set to....
If it is set to "Mute" then simply slide the switch on the side of your iPad and you will get your sounds back.
If it is set to "Lock Rotation", then double click the home button, swipe the icon tray to the right to reveal the "controls" panel, and press the mute button which is on the left hand edge of the screen.
Depending on how you have the setting set, one of these two will fix your problem.
It is worth noting, because you may think you dont have mute enabled because you can still hear some sounds, that Apple did a VERY STRANGE implementation of MUTE. Having mute on does NOT mute all sound like you might expect... So don't assume you are not muted just because you can here some things...
Hope that helps!