I'm going to assume here that Mikeyfish90210 is talking about making a mission with NO voice sounds, but with characters moving their lips anyway when they are "talking". Think of it like a silent movie, not a bad kung-fu movie. I've seen this in other missions, and it looks MUCH better than just having a guy sit there and "talk" with his mouth closed.
Now, to answer your question:
First, you need a wave sound to make into a lip file. Go here:
http://www.naturalvoices.att.com/demos/ You can copy and paste your text, and it will spit out a wave file for you.
Next, take that wave file and convert it into a lip file using wave2lip.
Now, you can only get a character to move his lips using the "say" command; which requires a sound file. Make a very small, blank ogg file for each of your "voices". Now just write it to your description.ext and walla! You're done.
I assumed that you know how to get voices and lip files to work normally. If you don't, then you should read the tutorials that have been suggested by SharkDog.