I am no lawyer, but I am fairly well-versed in legalese, and a musician to boot. hugolino is spot on in this case. Technically the use of the copywritten music on the server is sketchy at best, but since there's no money coming in that the owners of the copywrite would likely be interested in, it's safe to assume that a lawsuit is highly unlikely.
I would suggest that any person(s) currently running or looking to run a NWN PW look for either open-source audio that fits the mood of the server, or better (if possible and/or feasible) have original music written specifically for the server. This can be troublesome, though. I am currently in college for video game design with a focus on music composition/audio design, and I know that paying a professional/semi-professional composer can get pricey rather quickly.
I would be more than happy to offer my services to the server in the way of composing background music, and for free, but with the caveat that I am in school full-time and working full-time as well, so I would not be capable of meeting super strenuous deadlines.