No Sound from Speakers
Troubleshooting sound-related problems in Windows 7 can be complicated, due to the multitude of different sound devices there are on the market. Assuming you have checked the obvious, such as to see that all volume controls are turned up (both on the speaker system and within Windows) and all of the speaker cables are correctly connected, consider the following troubleshooting options.
1 - Volume Icon is Crossed Out
If there is a red cross over the volume icon, it means that Windows 7 is unable to detect a sound device on your computer, in which case there will be no sound or audio-related features at all. This is almost always the result of a faulty sound driver. Open your start menu and type "device manager" into the search box. Click the result which appears to open the Device Manager. You will see a list of hardware installed on your computer. Scroll down the list until you find the section "Sound, video and game controllers." Beneath this section, you should see your soundcard. The section will already be open if a problem has been reported. Right-click on your sound card and click "Update driver." Work your way through the wizard to update the driver. If the wizard cannot find any driver for your device, try downloading one from the manufacturer's website or installing the driver from a CD which came with the computer or sound device.
2 - Everything Appears Normal, Yet Still no Sound from Speakers
If the volume control icon is displaying full volume and everything appears to be normal in Windows, then the problem is likely with your speakers. Test the speakers with another computer or, ideally, a portable media player to see if they are working. If there is still no sound from the speakers, then the problem is almost certainly with the speakers.
If the speakers worked, then the problem is likely still with Windows. If you have multiple sound devices installed, this will almost certainly be caused by having the wrong device selected. Right-click on the volume icon in the system tray and click "Playback devices." Right-click on the device which the speakers are plugged into and click "Set as default device."