The keyword for your request is Digital Signal Processing (DSP). That is the science and technology that deals with digitized signals (including audio).
There are several freely available resources on the web to learn some basic or advanced techniques. Nevertheless, some concepts require a solid mathematic background.
Examples (among others) :
The Scientist and Engineer's Guide to Digital Signal Processing
DSP on coursera
Foundations of Signal Processing
You can actually do audio processing with many different languages depending on the context and the required performances. Real-time processing adds a layer of difficulty in terms of programming. If you are interested only in the MacOS platform, the Core Audio framework can be used.