Check that your device’s sounds are on and that the game sounds have not been muted. Also update to the latest game version if there is an update available. If you still have this problem, restart your device and try again.
Additionally, the side switch can be set to either mute sounds or lock rotation. It is possible that the sounds are muted with the mute switch, but the button itself is set to lock rotation. To restore sounds in this case, you will have to go to
iOS settings - General and under "Use side switch to:" select "Mute"
Now just mute the sounds with the side switch, launch Angry Birds and unmute sounds. Afterwards you can change the side switch back to rotation lock.
Some users have also reported that they got the sounds back by muting with the side switch, setting the game to mute, setting game sounds back on, and finally turning sounds back on with the side switch.
Another option is to mute the device sounds with the side switch, shut down the device completely, turn it back on and then unmute.