There are some issues running CoD 4 multiplayer on some onboard sound cards. If you have a RealTek sound card, you can try the
latest RealTek drivers
If you have a Sigmatel® onboard sound card you will need to install all Windows® Vista updates, and the latest Sigmatel drivers from
Learn about Dell's laptops, desktops, monitors, printers plus computer electronics & accessories.. Please navigate from the dell homepage to support, your make and model computer, and look for a sound card update.
If you are running the game from a secondary partition, download the latest BIOS updates from your motherboard manufacturer in addition to following the above instructions.
If you have the RealTek soundcard and the above doesn't resolve the issue then try the following:
Go to Start then Control Panel
Double click on the Sound Tool
Click on the Recording tab
Right click and select "View Disabled Devices"
Enable the Stereo Mix
If all else fails try connecting something to the microphone jack on your sound card. A microphone is optimal but using a pair of headphones or even just a phono plug adapter will work. Do not plug anything that is powered into your microphone jack as it could cause damage to your system.