This is the error message I am getting when I try to launch COD 4 Multi-player. Single player runs smooth.

Problem signature:
Problem Event Name: APPCRASH
Application Name: iw3mp.exe
Application Version: 0.0.0.0
Application Timestamp: 474cd49a
Fault Module Name: iw3mp.exe
Fault Module Version: 0.0.0.0
Fault Module Timestamp: 474cd49a
Exception Code: c0000005
Exception Offset: 00276959
OS Version: 6.0.6001.2.1.0.768.3
Locale ID: 1033
Additional Information 1: fd00
Additional Information 2: ea6f5fe8924aaa756324d57f87834160
Additional Information 3: fd00
Additional Information 4: ea6f5fe8924aaa756324d57f87834160

From reading your error message it appears that the multiplayer executable is at fault. Have you tried to reinstall the game? Ona side note, are you using any cracks?

I have reinstalled, and I am not using any type of cracks. I am on Vista 32bit home premium. 4gig ram, 8800gts 320mb, 6700 Core 2 Duo. Sigmatel On board sound drivers, Dell XPS 410.

Also, I plugged the speakers into the mic jack, and then the multiplayer works, but there is no sound when I plug it back into the speaker jack.

The game can't recognize if a jackplug is plugged in or not, so it's strange that the game would work if you plug in your speakers into the microphone port, as the two have nothing to do with each other. To me it sounds like your sound card drivers need updating or are corrupted. Reinstall those for a start. Once reinstalled, if it still doesn't work, you can try to reinstall the game. Make sure that your sound card is properly configured in your operating system as well as in your game. Kill all applications which are running while you're playing the game, as one of the other applications might try to use your sound card; just to make sure.

Well I have done all of the above. The activision's website say that this is a glitch/error or something like that on alot of on-board sound cards. They say its fixed. However its not. And most of the fixes for it, does not work.