Could be a number of things.
A little information about what kind of system you are on would help.
It's possible you've simply turned mute on. I can't really say with no idea what hardware you have.
My Computer
Computer Manufacturer/Model Number
Insane hobo technologies. ;-)
OS
Windows 7 x64
CPU
Intel i7 2600k
Motherboard
Asrock z68 extreme 4 gen 3
Memory
G.skill Ripjaw 16gigs @ 1866
Graphics Card(s)
Nvidia gtx580 (evga)
Sound Card
Integrated HD audio + hdmi
Monitor(s) Displays
24" ASUS widescreen + 42" insignia
Screen Resolution
1080p (1920x1080)
Hard Drives
128 Samsung 830
256 Samsung 840
3 x 1tb storage drive (various)
1 western digital 1tb (eSATA)
1 Seagate 1tb (eSATA)