1. A method for reducing speech intelligibility while preserving environmental sounds, the method comprising: receiving an audio signal; processing the audio signal to separate a vocalic region that comprises vowels; computing a representation of at least the vocalic region, the representation including at least a vocal tract transfer function and an excitation; replacing the vocal tract transfer function of the vocalic region with a replacement sound transfer function of a replacement sound to create a modified vocal tract transfer function; and synthesizing a modified audio signal of at least the vocalic region from the modified vocal tract transfer function and the excitation.