Not too sure, but anything short of removing your balls will be tough to emulate
Seriously though, you could experiment with speeding up your audio clip while syncing the video to match the new audio length. Try talking higher and softer during the recording, and then speed it up afterwards until you find the 'sweet spot' ... just stay below the 'alvin & the chipmunks' frequency though, that's just frikn annoying (if you're from USA you'll get my reference).
Just did a quick search, appears a program called 'av voice changer' can do it, they have couple editions.