program code for mp3 leverling

Hi
I new a bit of programming. Wher can i find code to read a mp3 file format so that i can increase of decrease the volume of mp3.

What?
Do you mean “I need”, or “I know”, or something else?

Audacity uses libmpg123 (libmpg123 download | SourceForge.net)
another option is libmad (LIBMAD)