Tempo/Tonality modification

I am a french student in an engineering school. I have to work on a project in which I have to modify the
tonality of an audio track without modifying the speed of the track and vice versa. That is why I would like to know
what are the methods and the algorithms Audacity is using to do these operations, or where could I get documentation about them.
Thank you.

You mean changing the “pitch” of sounds without changing the tempo?
Audacity does this using the “soundtouch library”. Soundtouch has it’s own web-site here: http://www.surina.net/soundtouch/