Shorten is a relatively old lossless audio format. It is no longer developed and has been replaced by the likes of the Free Lossless Audio Codec (FLAC) and Monkey's Audio (APE). Shorten file are available in many places on the Internet however, bearing the .SHN extension. There is not a lot of native media player support for Shorten audio content, but it is relatively easy to find out How to Play SHN files and how to Convert SHN to MP3 if need be.
Shorten was developed as a way to compress audio tracks from CDs without losing any quality. Because of the lossless nature of the Shorten format, if you do get any Shorten files, converting them to another format won't be any different than ripping content from CDs in terms of the output quality. Shorten is found widely used in many legal concert recordings on the Internet
More information: How to play SHN files, Burn SHN files to Audio CD with Nero, Convert SHN to MP3.