Somone get this? Maybe somone have this 5 songs from GOG?
What GOG has is mere snippets of the ambient tracks, less than a minute long. You're probably better just having a character sit idle, recording "What you hear" and making an mp3 off of that.
And no, I'm not giving you the tracks that GOG gave me.
Arx has no 'full length' tracks in any part of its in-game soundtrack, all it consists of are a number of rather sort loops.
Digital Nightfall made the effort of collecting them into a single zip which can be downloaded here. Given that Digi knows Raf (director of Arkane) personally I can only assume this collection is accepted to be publicly available..
There's also an application called Arxpak (http://www.ttlg.com/forums/showthread.php?t=122913) that allows you to extract files from the game's archive files. This will let you get all of the sounds and music in their original quality.