While I usually have the music turned down while playing, there are some pieces that are either evocative (the newbie zone music can take me back to my 1st days in Norrath) or just pleasant pieces (PoT is an obvious one). That said, Games Music rarely stands up on its own.
However, if you've been pre-downloading the GoD updates, browse to your eq folder and wrap your ears around 'Mountainpass.mp3'. Rob King and Paul Romero have really pulled one out of the bag here.
Trust me - if you fancy some chill-out/mood sounds, this is excellent.