If the GPL is used for art and music, these "source files" must be available. This could be beneficial, as it enables people to tweak and update the music (even if the composer has left). I could also imagine the score for the music alone qualifying as the source (although this can be difficult for some music which is never written, but only sequenced, and also might not be the "preferred form of the work for making changes in it"). Another benefit is that the score and resulting sheet music might be used for a professional orchestral performance, eliminating the use of a synthesizer (resulting in better quality music and more publicity for the composer). The strong copyleft of the GPL would probably require the orchestra to release sound recordings under the GPL, meaning that the recordings (if they would be of high quality) could be used in the game.
Another related issue is the use of proprietary samples. According to Brett Smith, from the Free Software Foundation Licensing Team:
The proprietary samples raise a separate issue, however: you need to be certain that you can distribute the music you create under the GPL at all. If the license prohibits you from creating modified versions of the samples, or from giving others permission to do so, you effectively can't distribute the music under the GNU GPL at all, since anyone with an GPL'ed Ogg of the song could do any of these things. You should check all the relevant licenses for your music software and samples to see whether or not they provide you with enough permission to release your music under the GPL.
A solution would be to release art and music under the Creative Commons Attribution Share Alike (by-sa) license (which doesn't solve the issue of unmodifiable samples) or the non-free/proprietary Creative Commons Attribution No Derivatives (by-nd) license.
My personal view is that maybe source files could be provided on a wiki page, at least to comply with the GPL. A possible side benefit would be the possible availability of sheet music, which might even be played by a professional ensemble or even orchestra in the future. The second issue regarding soundfonts and samples is relatively minor, and could be resolved later. It might even be inapplicable to music, or one could consider the samples to be a library (though this could be a stretch). As a last resort a Creative Commons license could be adopted.
Even music for other works, such as films (which are not as liberally licensed) have sheet music available. For example, you can buy Star Wars scores written by John Williams. It would be good to get the Wesnoth community to embrace the music beyond simply including it in the game. Even if Creative Commons Attribution No Derivatives (by-nd) is adopted for the music, non-modifiable sheet music can still be made available, but they could only be done by the composer.
Please realize that there are several issues here (maybe too many for one thread).
1. Source files to comply with GPL (major issue)
2. Modifying samples/soundfonts (relatively minor issue)
3. Sheet music for Wesnoth community to further embrace the music, for greater publicity, and the possibility of performance (which can be included in the game)