SHN is not a bit-torrent file type, it is a lossless music format called Shorten. Lossless means it has zero quality loss due to file compression (MP3 on other hand, has some loss). Shorten files are usually larger because of this.
Here's a winamp plug-in for it: http://www.etree.org/shnamp.html
If you prefer Windows Media Player theres problem something similar. I'll post if I see something.