(Solved) Filter/effect to replicate a megaphone

For a history project at school, my group would like to make an audio recording that sounds like what was played over the PA system during World War II (sounds kind of like a megaphone). Is there any free software out there that anyone can recommend which can give that megaphone effect to an audio recording?

thanks for any suggestions.

actually...thanks for help guys, but I've just messed around with iMovie and am happy with what it can do under the "effects" tab. Garage bands sounds like a great idea, but I'm not much of an audio person so I actually uninstalled Garage band a while back (not worth the trouble reinstalling when I am happy with iMovie's audio effects).

thanks again.