This site describes several different computational approaches to acoustic echo cancelling. This online paper examines theory and techniques for noise reduction in audio processing. The first few sections are good introductions for engineers wanting to learn about noise introduction.
Later sections become more detailed and provide excellent examples of noise reduction techniques.
This site is a paper comparing various lossy audio compression techniques for fidelity. It also contains links to free downloads for various encoders and decoders. If you're looking for simple algorithms to code audio effects, without having to look it up or design it on your own, check out this page. It provides actual C code, describing its use, the required parameters, and limits of each algorithm. The page includes various effects on amplitude, time delay, waveform distortion, and frequency response.