The only way you can really do this without having access to FBI resources

is to use audio editing software (
Audacity is a very good free sound editor) to chop up a spoken sample and rearrange the bits to form a new sentence.
Of course you'll be limited to whatever spoken words there are and you can't change the way they're spoken...at most you could do some trickery with quick pitch changes and the like, but the result will be far from natural.
Consider this...time-stretching is a very popular sound processing technique and basically means altering the speed without altering the pitch. This is typically done by chopping the sound into tiny bits and repeating some of them so as to artificially lenghten the sound (or skipping bits to shorten the output) but keep the pitch. There are decent algorithms to do this with very little artifacting.
There's one way to do a
perfect timestretching, but it's computationally incredibly intensive. Basically it means "disassembling" the voice into its component sine waves - every sound, no matter how complex, is ultimatively just an addition of sine waves of all kinds of frequencies and amplitudes - and then resynthesizing the sine waves with different time lengths. This process is known as FFT (Fast Fourier Transformation).
A few years ago, some geeks processed a sample of a spoken phrase in French that lasted 1.5 seconds. It took the computer several
hours to create a perfect timestretched copy.