Filtering for example. If you are dealing with audio data, it is worth familiarizing with Fourier analysis to a good proficiency level. I'll give you some links.

http://ptolemy.eecs.berkeley.edu/eecs20/labs/LabVIEW_Labs/Lab07/Lab07.pdf

http://www.dspguide.com/pdfbook.htm

http://www.amazon.com/Digital-Audio-Signal-Processing-ouml/dp/0470997850

S.J. Godsill and P.J.W. Rayner. Digital Audio Restoration - a statistical model-based approach (Berlin: Springer-Verlag 1998)

Ivan J. Tashev. "Sound Capture and Processing: Practical Approaches", Willey 2009 - Chapter 4.

I believe you are looking at Active Noise Control. The field of ANC has 2 camps: (1) the digital signal processing approach and (2) the acoustics approach.

For the DSP approach, Prof. Kuo provides a very beginner-friendly approach with attached C code on the algorithms.

• S. M. Kuo and D. R. Morgan, Active noise control systems: algorithms and DSP implementations: Wiley, 1996.

Additionally, the book from Lee et al. has included MATLAB codes of the algorithms used in ANC and even an Appendix for MATLAB.

• K.-A. Lee, W.-S. Gan, and S. M. Kuo, Subband adaptive filtering: theory and implementation: John Wiley & Sons, 2009.

For the Acoustics camp, the book by Prof. S. J. Elliott covers basics from acoustics wave equations to matrix formulations, he has one of the most publications in the field of ANC in recent times,

• P. A. Nelson and S. J. Elliott, Active control of sound: Academic Press, 1993.

His follow up book focuses on signal processing techniques:

• S. J. Elliott, Signal processing for active control: Academic press, 2000.

Another group of researchers working in the acoustics realm are:

• C. H. Hansen, Active control of noise and vibration vol. 1: CRC Press, 2013.

If you are looking for to perform noise reduction on speech signals I think the best book to start is:

P. C. Loizou, "Speech Enhancement: Theory and Practice". CRC Press, 2013.

It comes with a DVD full of code examples and a database you can test your algorithms on.