External speakers running on a PC must be plugged in two different locations: (1) with a DC power supply to an electrical outlet and (2) with an speaker cable plugged into the Audio Out port on the back of the computer. If the cable isn't plugged into the Audio Out port, this could be why you have no sound.
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Are the Speakers Plugged In?
Check Your Settings
In the Windows XP Control Panel, double-click the "Sounds and Audio Devices" icon. In other operating systems, this could be labeled "Audio Settings," "Volume Settings," etc. There should be an icon of a speaker, either way. The Sounds and Audio Devices properties window will open on the Volume tab. Check the device's volume settings. Is it set to a low level of volume? Is the "Mute" box checked? If one or both of these are true, that could be your problem.
Next, click the "Speaker Volume" button found in the "Speaker Settings" section below. If the slide indicators for the right and left speaker are at a low volume level, increase the level.
Finally, click the "Advanced" button for the "Speaker Settings" and select the type of speakers you have in the "Speaker Setup" drop-down menu. If the menu is set to "No Speakers" or any other type of speakers than the ones you have, you're not going to get audio. Change the settings and see if this gets your speakers to work.
Check the Volume Levels
Check the volume settings on the speakers and make sure the volume knob is turned up. Check the settings in the volume control on your computer, to make sure the master volume isn't muted as well as the CD volume and line out volume. If these are muted, that could be the reason the speakers are not working. Likewise, check the volume settings on the software you are using. If the volume level is turned down in Windows Media, iTunes or whatever you use to play music, you won't hear anything. Also check your audio configurations in these programs to exhaust all avenues of what might be wrong.
Once you've exhausted all easy-to-fix solutions, check your audio driver. Right-click "My Computer" on your desktop and select "Manage." When the Computer Management window opens, select "Device Manager" from the left window pane and scroll down until you find your "Sound, Video and Game Controllers." Click this and select your audio driver(s). Make sure your audio driver is installed and check to see if there are updates. You can do this by going to "Action" > "Update Driver" in the task menu at the top of the window.