I like your enthusiasm!
Well for MP3 decoding there is the
MAD library which is used in a lot of free music players, time-stretching/pitch-shifting code is available via either the
SoundTouch Audio Processing Library or the DIRAC LE Library which is available at
http://www.dspdimension.com. DSPDimension also has a lot of indepth information on this subject - more than anyone except real DSP freaks will ever need.
Good Luck