.WAV files are sound files. All of MTM2's commentary and sound effects are in .WAV format.
.WAV files can be edited with a program such as Windows Sound Recorder. If you have a microphone you can record your own .WAVs, but if not you can still use this program to make your own commentary sound files for your truck out of existing commentary.
You will note that all MTM2 truck commentary filenames are followed by either a "d", "f" or "u". This is because the .WAV files have different accents on them, so when Army Armstrong starts throwing around his commentary phrases, the .WAV file will match the tone of his voice. There's only ever a slight difference in the way he says it, but it's something the designers put in.
When the game picks a commentary .WAV involving a truck, it picks the appropiate truck .WAV to match it. So you get completed phrases like "Bigfoot is doing it in the air!". "Bigfoot" and "is doing it in the air!" are actually two seperate files. This way the designers would need only one .WAV for each commentary phrase, and each truck would have its own commentary .WAVs which are inserted into the phrases. It would also allow people making addon trucks to give it it's own .WAV, and the game would use it with the phrases to make it sound like Army was really commentating on that truck.
Unfortunately not many people sound like Army Armstrong, so there is a problem in making your own .WAVs for your truck.
The simplest way is to find an already existing .WAV for use with your truck: this can be an existing commentary file (like the Bigfoot .WAVs) but it doesn't have to be, it could be from an entirely different source if you want (using Warcraft II .WAVs never fails to amuse). Of course, you may not find a .WAV that actually says your truck's name, so you might have to settle for the game calling it something else (like "Bigfoot").
The second way, if you have a microphone, is to record your own voice. This often sounds less than genuine in the game, as the commentator's voice will sound like it changes every time he mentions your truck! However, it is the easiest way of actaully getting him to say the right name for the truck.
The third and hardest way is to use a program like Windows Sound Recorder to dissect and reassemble existing Army commentary and reassemble it to make your truck's name. The program lets you delete and insert sound before and after the slider's current position, so you can cut up a few .WAVs to get a certain sound, then stick them all together. This can be easy or hard depending on your truck's name. For example, for my Nissan Big 1 replica, I made a .WAV that goes "Big One" by cutting "Big" out of a "Bigfoot" .WAV and adding "one" cut from a commentary .WAV that goes "There can only be one". That was an easy one. A hard one would be my Devil's Dodge replica: since army never actually says those words, I had to improvise and came up with something sounding a bit like "dhEVERs DHUGe", which was as close as I could get. Often with words like that, you have to break it down into syllables and find .WAVs with those syllables. Sometimes they'll flow together and make something that sounds convincing: sometimes they won't.
Or you could simply give the truck no commentary .WAV. If the game can't find it, it won't use it. This means you will never hear Army mention your truck other than those commentary phrases where he doesn't actaully mention the truck's name. Of course, some people hate Army's commentary and have him turned off, so making a commentary .WAV would be a waste of time.