Hi Ramia, I'm definitely a newbie, but I did install Ubuntu recently and mute was checked by default. I know you aren't using Ubuntu, but it might be worth really digging around in the settings menus to see if something like that might be happening. It took me awhile to find the problem.
I also wanted to tell you that I downloaded a typing tutor that had a "sounds" box checked in one of its menus by default. Even when the program wasn't running, I would get a robot voice telling me exactly what my mouse was hovering over. It was very interesting but also annoying. I thought it was Ubuntu's Universal Access (settings to help people with disabilities) malfunctioning, but the little typing program was just able to override all the Ubuntu settings. The robot didn't go away until I unchecked that silly box.
Anyway, I hope reading about my experiences gives you some new ideas about where to start looking. I have definitely learned that I have to keep up with all the programs and understand their menus, etc before downloading more. I got a little excited when I first got Ubuntu and figured out to do some basic installation.
Also, have you looked around in your area for a linux user group? I happen to live in Raleigh NC where Redhat is, so I'm really scared to go to a meeting, but I've decided to try one. Anyway, best of luck!
Machine: Toshiba Portege r705-P41, Dual Boot: MS Windows 7 Home Premium 64-bit; Ubuntu 15.04
CPU: Intel Core i5 460M @ 2.53GHz Arrandale 32nm Technology,
RAM: 4.0GB Dual-Channel DDR3 @ 532MHz (7-7-7-20), Motherboard: TOSHIBA Portable PC (rBGA1288 Socket)
Video Card: Intel HD Graphics Revision 2 1720 MBytes