Problem with my power button

[fonzejaa]

Hi all, I recently started facing a problem with my Lumia , when i hit the power/lock button to the sleep the mobile the sound that i used to hear when i hit the button doesn't appear anymore !
any help ?

 

[DBDev]

Do you have the ringer volume on or just vibrate?

 

[fonzejaa]

I have checked my mobile now and it gives me sound now but i tried to lock and open it again and the sound is gone again !!