Sounds like you are getting the IW3MP.EXE error m8, it is a sound error and there are many possible fixes, which you have already discovered one ie having headset plugged in, most get this error if they have Realtek soundcards, you could try updating your sound drivers and see what happens, if you have just updated your drivers and this problem started just after that then roll drivers back, also you could try to go into your Activision\cod4\miles folder and change the mssmp3.asi file to mssmp3.bak, these possible fixes work for some and not for others, the only thing that works for me if the problem gets really bad is to uninstall Cod4 then update your sound drivers, then reinstall game back to the new drivers, a bit drastic but it always works for me.
tried changing mssmp3 file didn't work my try he uninstall, do I lose everything by uninstalling?