This is my standard checklist for audio troubleshooting. If you have not installed any new hardware, item (6) does not apply.
There are several possibilities. Each requires a brief diagnostic. For a sound source, click Start, Run, enter "DXDiag" and click OK, then click Sound, Test DirectSound.
1) Most speakers now available require a power supply. If yours does: a) Verify that it's connected; b) Connect one end of the audio cable to the speaker amplifier input (not the speaker input), and the speakers to the amplifier outputs; c) Turn up the amplifier volume, wet your finger and touch the tip of the free end of the cable. If you do not hear a hum or buzz, the cable, speakers, or power supply is faulty.
If your speakers are unpowered, read the manual to see if the soundcard can drive them. Most current soundcards can't.
2) Connect the audio cable between the speaker amplifier input and the soundcard's front speaker output (the jack is often identified with a green ring). Run the Volume Control program in Start >Programs >Accessories >Entertainment (if it's missing, install it with Start >Settings >Control Panel >Add/Remove Programs >Windows Setup >Multimedia >Volume Control). Verify that Mute is NOT set for any input, and that the sliders are up to at least the 5th mark.
3.0) Go to Start >Settings >Control Panel >System >Device Manager, and expand Sound, Video and Game Controllers. If there are duplicate entries for the soundcard, remove all but one. If an entry is marked with a yellow ! or red ?, select the device to determine why, and see any suggested solution. Double-click the soundcard and verify "Disable..." is NOT ticked, and "Exists..." IS ticked. If your sound is listed in another device section (miscellaneous devices, perhaps), verify that Disable and Exists are also set as described above.
3.1) Still in Device Manager, expand System Devices and PCI bus, then click the IRQ Steering tab. Tick the "Use IRQ Steering" check box if it is clear.
If you've made any changes, restart the computer for them to take effect.
4) Go to Start >Settings >Multimedia >Audio, and verify your soundcard is the preferred playback device. If the soundcard is not specifically listed, choose "Game Compatible Device". Tick "Use only preferred devices" and "Show volume control...". Click the Devices tab, double-click Audio Devices and your soundcard. Verify that "Use audio features..." is selected.
5) You may need to reinstall the soundcard driver. If an updated driver is available, you should try it.
6) Click Start, Run, enter "MSINFO32" and click OK to launch the System Information utility. Double-click Hardware Resouces to expand it (if necessary) and click Conflict/Sharing. If the soundcard is sharing its IRQ with another device (except as an IRQ holder), you may have to move the soundcard, or the other device, to another PCI slot so that the soundcard does not share its IRQ.