In this tutorial we use a tool called Audacity which is provided free.
Go ahead and open your source file, audacity can handle a lot of input files. Once open you'll notice the track and a waveform of the current file.
First off at the bottom left of your screen change the sample rate to 44100, this makes the project setting change to the rate we need. See below for an example.
Next go to the arrow at the top left of the current track and select Rate->44100.
Note: If your sound file is already mono, you can continue to saving. Otherwise follow along below.
To convert from a stereo to a mono track you need to go to the arrow again at the top left of the current track and press Split stereo to mono This will create two separate tracks for you.
Now select both tracks using shift and go to Track->Mix and render, you will now see one combined track that will be mono.
To save the file simply go to File->Export Audio, select Wave and then signed 16 PCM, This will provide a medium quality audio file.
You'll be presented with another dialog for file tags but be sure to ignore them and just continue, the file will have saved and it's ready for Call of Duty: World at War or conversion using the Black Ops sound tool.