Audacity - audacity.sourceforge.net
There are plenty of free audio editors available that will allow you to create and edit multi-track files. Audacity is easy to install and easy to use, but don't forget to install the LAME encoder if you want to export your work to MP3.
Finding Audio ResourcesThere are lots of fantastic resources for finding audio samples to work with. I've linked a handful below:
ActivityIn class we are going to take some time and familiarize ourselves with the Audacity interface. To help get us started, I've linked some audio files below to download and import into Audacity:
Example StoriesThese examples aren't all from this class, but are interesting and similarly-themed. I really like the first example (although it is a little on the existential side) because the creator shares a bit about his process and notes all the audio sources he's used, along with links back to where they were downloaded from.