A very popular standard for compressing audio and particularly music files down to a reasonable size with little or no perceptible loss of quality, and the files created using it - "an MP3" is an audio file.

Since it is relatively easy to create MP3 files from CD selections and make them available on Web sites for downloading, companies and sites that promote the MP3 format are sometimes accused of encouraging copyright violations. (It is illegal to copy music from a CD and redistribute it unless you have the copyright owner's permission.) On the other hand, MP3 enthusiasts claim that what CD publishers are afraid of is any kind of non-CD distribution. While there are several proposals for how to discourage such piracy, there is currently no secure distribution and copyright management standard that publishers and other parties agree upon.

Several Web sites are promoting MP3 as both a high-quality audio format and as a way in which self-publishers can gain ready access to an audience. Currently, some music publishers are providing sample cuts in the MP3 format as a way to entice users to buy a CD. However, not much mainstream copyrighted material is available except as an illegal download.