- Bad or missing drivers
- Faulty hardware
- Software misconfiguration
Bad or missing drivers
This usually appears as a sound device with errors or simply sound device with no driver in Windows Device Manager. If you have HD audio hardware, then it might appear as Unknown Device if there's no driver installed. Be sure to check under the sound devices section to be sure an unknown device isn't something else, or you'll end up on a wild goose chase.
Whether it's incompatible drivers, corrupted drivers, or missing drivers the solution is generally the same: install the driver using the sound driver installer for the hardware. This can usually be obtained from the motherboard manufacturer, OEM system builder or sound card maker. This can be a very frustrating exercise if you have a very rare sound card that none of the usual suspects provide drivers for. Old Intel AC'97 and Conexant AC'97 audio comes to mind.
There's nothing much to do here except verify whether it's the speakers or the sound card that's faulty. Replace the faulty hardware and be sure the driver's properly installed if the sound card was replaced. Be sure it really is faulty hardware and not just the speakers turned off or plugged into the wrong jack.
This is by far the most difficult to deal with. Various software related problems can have the same or similar symptoms which makes it harder to pin down. A panicky user who can't even describe what's going on or read back an error message merely serves to compound the problem.
The most common software related problems would be muted sound volume, or incorrect default sound device. In the first case, the simple act of increasing the volume, or removing the check mark for the Mute option would remove the mute status (see highlighted above) while in the second you'll need to select your sound card as the default device under the Audio tab in Control Panel -> Sound and Audio Devices (see below).
For something a little less common, the sound card was disabled in Device Manager. Just enable it from the context menu (right click on the device) in this case.
The rarest I've encountered would be caused by a disabled Windows Audio service. If the Windows Audio service was disabled for whatever reason then you can be sure that there will be no sound whatsoever regardless of the condition of your hardware, or drivers. You will not even have volume control available.
For this, you'll need to head over to Start -> Run..., then type services.msc as shown in the image above. Find Windows Audio (highlighted in red box) and double click on it. You'll notice that Startup Type is set to Disabled and there's your problem there. Just set it to Automatic and reboot your computer to resolve the problem.