Violet Sedan Chair is a fictional psychadelic rock band created for Fringe and is Walter and Walternate's favorite band. In the original timeline (seasons 1-3) Walter's actions caused a butterfly effect that led to the band breaking up in the Blueverse after releasing only one album whereas they released at least three albums in the Redverse. In the new timeline without Peter (season 4) the band released at least two albums in the Blueverse.
Despite being a fictional band, real musicians (identities unknown) were hired to write and record an album as Violet Sedan Chair. A small number of vinyl records for that album, Seven Suns, were pressed and sent to independent record stores across the U.S. It has been rumored that some records were even weathered so that stores would put them right into 'used' bins to make them harder to find. And each individual copy of the album may be unique in some way, featuring specific spoilers and clues about the show. Many records have been found but there's no way to know how many may still be out there. The album can be played for free on YouTube, however (see below).
In April 2009, six months before Violet Sedan Chair was first mentioned on Fringe and nearly two years before they were featured as a plot point, J.J. Abrams was a guest editor on a wired.com article called "Musical Mystery Tour: Messages Embedded in Your Favorite Album" where he snuck in a mention of Seven Suns as if it were a real album. The record sleeve shows the final track to be "Greenmana" but this track doesn't actually exist on the album.
Listed below are all the references to Violet Sedan Chair throughout Fringe, videos about the band of mysterious origin, and a playlist to listen to Seven Suns.