Hi there-make sure that the side switch is not set to mute.
Go to settings/general/side switch. See if it is the mute option is checked and uncheck it.
Out of the ios5.1 Manual...
Side Switch
You can use the Side Switch to disable audio alerts and notifications. You can also use it to lock the screen rotation and prevent the iPad display from switching between portrait and landscape mode.
Mute notifications, alerts, and sound effects
Slide the Side Switch down to mute notifications, alerts, and sound effects. This switch doesn’t mute audio playback, such as music, podcasts, movies, and TV shows. See Side Switch.
Lock the screen rotation
In Settings, go to General > Use Side Switch to, then tap Lock Rotation. See Side Switch.