Just plug something into your mic jack or enable "Stereo Mix" in the Control Panel.

• Go to the "Control Panel".
  
• Go to "Hardware and Sound".
  
• Then double click on "Sound".
  
• Then click the "Recording"-tab.
  
• When you have done that, right click and then select "Show disabled devices".
  
• Then when stereo mix comes up, enable it and it will work fine.

I know this sounds weird, but it does work.

CoD4 uses PunkBuster for anticheat. First of all, the Steam version of CoD4 is delivered with an outdated version of PunkBuster. To make things harder, EvenBalance has stopped supporting PunkBuster for CoD4 and aren't distributing the cheatdefinition-files anymore. However, you can still get PunkBuster working on CoD4 by using the last distributed definitions.

• Grab the latest cheat definition files by following the link provided in this thread: http://gtm.steamproxy.vip/app/7940/discussions/0/35219681488541328/
  
• Unzip the archive, and move the two directories to "C:\SteamPath\steamapps\common\Call of Duty 4\PB". Replace SteamPath with the actual path to your Steam installation folder.

You should now have the latest cheat definition files, and PB should hopefully work for CoD4.

The PBSVC utility may be used for checking for errors related to PB, and it's communication to the PB server.

• Download pbsvc.exe from Even Balance[www.evenbalance.com].
  
• Run the .exe with administrator rights, and follow the on-screen instructions.

http://www.evenbalance.com/downloads/pbsvc/pbsvc.exe

Edit: I believe the bug related to the Steam overlay has been patched. However, I haven't got around to verify this yet.

That's a problem with PunkBuster. It flags some overlays (i.e. Steam In-Game Community) as a cheat, because this is the way most cheats work. Disabling Steam In-Game Community should fix the problem.

• Open Steam.
  
• Goto "Library".
  
• Right-click on "Call of Duty 4: Modern Warfare".
  
• Goto "Properties".
  
• Uncheck "Enable Steam In-Game Community".

If this doesn't work, check if you're running some other overlays, i.e EVGA Precision and disable them.

This is due to an error with the CoD4 masterserver and the servercache.dat.

Here you have a fresh copy of the .dat-file. Place it in the "C:\SteamPath\steamapps\common\Call of Duty 4\" directory

{LINK REMOVED}http://www.mediafire.com/?nc1sxly1nqtt6yi
https://www.virustotal.com/file/82682a7f263765829c6ae812f184edd363ece78c78b94219f226d111e767b256/analysis/1352992898/

You could also try to set the file to "read only".

The CDKey is stored in the registry. To view it, do the following steps:

• Press Win+R.
  
• Type "regedit".
  
• Navigate to "HKEY_Local_Machine".
  
• Go to "Software".
  
• Go to "Wow6432Node"
  
• Navigate to "Activision".
  
• Click on "Call of Duty 4".
  
• The key should be stored under the entry "codkey".

If you can't find it, try to update PunkBuster for CoD4, start the game and then do the same steps as stated above.

If you can't find the key at all, you most likely will need to re-install the game, as that would make a new entry in the registry.

The only difference is that the GOTY version includes the Variety Map Pack out of the box. "CoD4: Modern Warfare" users have to download it manually.

Steam only sells the GOTY edition, so Steam users don't have to do anything to use the Variety Map Pack.

The only difference is that the Steam-version patches the game automatically.
Remember that both versions might have to update PunkBuster manually though.

If you still got errors and problems, please verify your game cache.

• Go to Library
  
• Right-click on Call of Duty 4: Modern Warfare
  
• Settings -> Local files
  
• Verify integrity of game cache.