is there an App that can extracts parts of a mp3 file without re-encoding the file?
Sounds like Rogue Ameoba's Fission should do what you want:


  With Fission's streamlined audio editing, you can quickly copy, paste and trim audio, as well as split files. Fission also works with compressed MP3 and AAC formats to edit without the quality loss caused by other editors. Get perfect quality audio when editing natively in the MP3, AAC, Apple Lossless, AIFF and WAV formats.