The latest result of the Kerbal Sound Overhaul Project, this mod allows you to add and remove tracks from KSP's playlist for space and construction, as well as replacing the background music and ambiance in the other scenes. Currently .wav and .ogg filetypes are supported, as well as .mp3 on Windows (still working on a fix for Linux and Mac, but .wav and .ogg work fine). A selection of free stockalike tracks are included from musician Kevin MacLeod, who created many of the tracks used in stock KSP. Several thousand more tracks are available for free at his website here. If you find any that work well with KSP, leave a message and I'll add it to a future sound pack. A replacement for the Kerbal Space Centre birdsong ambiance is also included, edited from the original recording here. Once music files are dropped in the KSP\Music\ directory, the next time you restart the game, the files will be made available in the in-game playlist editor. This plugin makes use of MPG123 and associated code, licenced under GNU LGPL v2 (included).
Forum thread:
http://forum.kerbalspaceprogram.com/threads/56405