Hmm, A little off topic here since I do not even think that GoG sells NWN2.
Futures Questions on NWN2 would get better and faser responces at: http://social.bioware.com/forum/1/subindex/156
Anyway, To answer your question.
A .ssf file is a Sound Set File
It is not a music format. SSF are a list the ResRefs of Sounds to play and a refferance into a talk table for the text that appears over the players head. The list is for the emotes in the game. thngs like "greet" , "cuss" and combat sounds. Like when they grunt in combat.
EDIT: In Biowares words.
A soundset is a set of sound files to play and associated strings to display when a creature or player
character (PC) performs certain actions or when the creature or PC reacts to certain conditions. For
example, when a PC attacks a creature, the PC may shout a battle cry, with the text of the battle cry
appearing over the PC's head and in the game's message pane. The soundset tells the game what string
to display and what sound file to play.
/ End EDIT;
To get your music to work you need to convert it into .bmu format: MP3toBMU
An application allowing bulk conversion from/into MP3/BMU format music files.
The link was taken from B_Harrison's topic on custom content tools. Custom Content Tools