If all of your volume controls are OK, it is possible that your audio codec got turned off somehow. If you have XP, these instructions might be out of date, but for older editions of Windows you can try this:
Enter the BIOS by turning on the computer, then when the first logo screen appears, tap the F1 key repeatedly until a BIOS Setup screen opens.
Use the Left and Right Arrow keys to select the Advanced tab.
Press the Down Arrow key to select Onboard Audio Options and press Enter. If Onboard Audio Options is not available, select I/O Device Configuration and press Enter.
If On-Board Sound, Audio, Audio Codec or similar item is listed, select it and press Enter.
Select Enabled and press Enter. (If it's on Disabled, that's your problem.)
Press the F10 key and then press Enter to save and exit
Start Windows and check for sound.