im quite sure that you are using windows vista....
ive had the same problem..
so what you do is go to the call of duty 4 folder then to the "miles folder"
then look for the file called mssmp3.asi
Things to do
*change the mssmp3."asi" to mssmp3."bak"
*right click to the desktop
*click personalize
*click sounds
*click recording tab
*right click the big box
*click "show disabled devices"
*right click stereo Mix
*click "Enable"
*you live happily ever after
*you give me your bank account
hope it helps