1- You can fix it by simply plugging in a microphone or you can just plug anything into the microphone jack.
2- In the Activision folder, select Call of Duty 4.
Open your Miles Folder
Look for the Mssmp3.asi file
Rename it to Mssmp3.bak
3- Enable Stereo MIX, Vista RealTek
-Start, control panel
-Hardware and Sound
-Right Click in window, chose "Show Disabled Devices"
-"Setero MIX"appeared, Right Click and choose enable
These fixes should fixed your problems.