Another option besides Audacity is Goldwave
It also has a trial period. It also has several filters and effects such as Mechanize, Low/Highpass, Echo, and you can even change the speed the recordings with altering the pitch. (Of course, you can alter the pitch of your voice as well.)
In addition to Goldwave, you could always fool around with the free site of AT&T Laboratories. Go the AT&T Natural VoicesŪ Text-to-Speech Demo
page and enter the text you'll need. There are several voices to choose from. You'll have to record several files in a row - there's a limit to how much text is translated to speech at a time but you can record over and over again. In addition, you can always fool around with either Audacity or Goldwave to alter the results even further.
If you're looking to create a sound file so a character sounds like they are talking with a helmet on - like Canderous, for example - I'd suggest this page
at Massassi Temple for a decent walk-through.