Read the Attribution.txt file for more information. The majority of DSOUNDFONT Ultimate is under GPL, but some parts are under the MIT license. This is NOT a non-free artifact.
As a special exception, if you create a composition which uses these sounds, and mix these sounds or unaltered portions of these sounds into the composition, these sounds do not by themselves cause the entire composition as a whole to be covered by the GNU General Public License. This exception does not however invalidate any other reasons why the composition might be covered by the GNU General Public License. If you modify these sounds, you may extend this exception to your version of the sounds, but you are not obligated to do so. If you do not wish to do so, delete this exception statement from your version.