Before anything else, check if your speakers are all right. Plug them into a power socket, and see if they’re linked in properly. Use any type of audio device — portable or otherwise, to check if your speakers work with those. If they don’t, it’s probably time to get some new speakers.
Now make sure the speakers are connected properly. This rarely happens with 2 or 2.1 speaker systems (with all the colour codes and only one connection to make), but it’s easy to go wrong with a multi-channel speaker system. Read the manual that came with your speakers to make sure you’ve plugged them in right.
If you’re trying to use a multi-channel (say, a 5.1) speaker system with an onboard sound card, using the microphone and linein jacks as output sound channels, make sure that the sound chipset on your motherboard supports being used that way (Line out for Front, Line In for Rear and Microphone In for Centre). To find out which sound chipset is on your motherboard, open up your PC and look at the chip next to the sound jacks; note the name of the manufacturer and model number of the chip. Visiting the manufacturer’s Web site will tell you whether the chip supports connecting a 5.1 speaker system. Most new motherboards come with chipsets that support this feature.
If you’ve connected your multi-channel speakers, checked all the connections, and one or more of them still stay silent, check the power connections—make sure that they are secure and that each speaker is getting power.
If some of your speakers still stay silent, it is possible that your system thinks that you are using a stereo speaker setup. To change this, go to Control Panel > Sounds and Multimedia Audio > Sound Playback > Advanced, and select your speaker configuration from the dialog that appears.