Phone is stuck in silent mode

Press and hold the pound key (#) until it exits silent mode. 

Here is the original text from the manual of the phone:

"Quickly Switching to Silent Mode
Silent mode is convenient when you wish to stop the
phone from making noise, in a theater for example.
In Idle mode, press and hold the "#" key until the “Silent
Mode” message and the Vibration icon display...

To exit and reactivate the previous sound settings, press
and hold the # key again until “Exit Silent Mode” appears.
The Vibration icon no longer displays."

You can also download the pdf file for the phone's manual online by simply googling "Samsung x427m pdf manual."

Hope this helps!