You can analyze the frequency components of a recorded sound and then resynthesize—reconstruct—a representation of the sound using additive techniques. By calculating the frequency and amplitude of each harmonic in the overall frequency spectrum of the sound, an additive resynthesis system can generate a series of sine waves, with appropriate levels over time, for each harmonic.
After the sound has been resynthesized in this fashion, you can adjust the frequency and amplitude of any harmonic. Theoretically, you could even restructure a harmonic sound to make it inharmonic, for example.