Hey!
I'm in broadcasting in college in Ontario, and I have, as a matter of fact, changed voices.
The only drawbacks are a) if you don't use mp3 sound, you'll have FAR too big a file. Long download times aren't fun fun for anyone. and b) the files will likely sound like crap unless you have professional equipment. But it's still fun to do, and you can amaze ANYONE with a little bit of voice work
All you have to do is create all the osunds separately, and organize them - if you say "hi" into the mic, then that's it. Save the file as "YourNameHello1" and continue. If you've ever looked at the way Blizz saved all of the sounds, you'll know that there's "FootmanHello1", "GruntAttack3", "HeroPriestessPissed2" etc.
Pick a character's sound that you don't plan on using. For every "Hello" it possesses, do one of your own, etc etc until you've gone allll the way through.
Bob27 wasn't wrong when he said that you could use an outside program, but the Windows Voice Recorder will do; mind you, low-quality sound won't attract many people to your game!
All you have to do, after picking a model character, is right-click on one of the files in the Sound Editor. Select "Replace Internal Sound" and then pick your own file out: that's where the organizational stuff comes in. Just replace all or as many of the files as you can, and then when in the Object Editor, select a Model's Sound as whatever you changed, and you're good to go.
Hope that helps!
Chaotica.