When I press the + and - buttons, the volume icon appears, but the slider is empty. When I receive a call, I hear it ring, and I can hear the ringtones in the sound menu. However, the message/mail notifications don't make any noise, and I can't hear the s

When I press the + and - buttons, the volume icon appears, but the slider is empty. When I receive a call, I hear it ring, and I can hear the ringtones in the sound menu. However, the message/mail notifications don't make any noise, and I can't hear the sound menu.

- During a phone call, if I put the conversation on speakers ==> it works and the volume bar reappears. I can control the volume with + and - buttons. But when the conversation is over, the bar disappears again.

- With the headphones, everything is ok!

- I also noticed (only a few times) that when I do a silent to ringer mode, the volume slider reappears and works correctly during 2 or 3 seconds, before disappearing again.

I don't know what is problem,because the speakers work.