Fancy turning a kittens meow into a lions roar and back again, yup me to. Sound Morpher can help, and even better it is free! Sound morphing is the sound synthesis of two seperate sounds into one. The result possesses features of both seperate sounds at the same time.
http://www.elisanet.fi/victorx/SoundMorpher.htm make sure to click on the picture at that url to download it.