Audacity: to process mp3 files

Needed to filter humming noise from a lecture audio file, and found this amazing tool!

The references I used to learn it:

http://askubuntu.com/questions/429480/how-to-improve-the-sound-quality-of-an-mp3-speech-file
http://audacity.sourceforge.net/help/faq_i18n?s=editing&i=menu-disabled
http://www.youtube.com/watch?v=ZqxQc_8X4hw
http://manual.audacityteam.org/o/man/faq_opening_and_saving_files.html