These files have been encoded with FFmpeg to different codecs at several bitrates to see the difference between them. Some encoders that are built in can't encode audio at such low bitrates like libmp3lame or FFmpeg's native AAC encoder. However, codecs like Opus or Vorbis (with libopus and libvorbis encoders respectively) are somewhat designed for low bitrates.

The song I decided to use was Phoenix by Netrum & Halvorsen because of its royalty-free nature. The files I provide are available to be listened and used in projects (ones that feature audio and it has to be listened), since the song itself is free to use, as long as it's credited with the following:

Song: Netrum & Halvorsen - Phoenix [NCS Release]
Music provided by NoCopyrightSounds
Free Download/Stream: http://NCS.io/Phoenix
Watch: http://youtu.be/yH88qRmgkGI