/usr/share/supertuxkart/data/gui/skins/ocean/License.txt: This work is licensed under the Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/ or send a letter to Creative Commons, 171 Second Street, Suite 300, San Francisco, California, 94105, USA.

/usr/share/supertuxkart/data/gui/skins/peach/License.txtThis work is licensed under the Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/ or send a letter to Creative Commons, 171 Second Street, Suite 300, San Francisco, California, 94105, USA.

In /usr/share/supertuxkart/data/sfx/License.txt there is a reference to SoundSnap license (short version):

|    You are Free:
    |        - To remix or transform the sounds in any way
    |        - To copy, distribute and transmit the sounds
    |        - To use the sounds in any music, film, video game, website etc. whether commercial
    |          or not, without paying royalties or other fees
    |    You Cannot:
    |        - Make commercial distribution of these sounds 'as they are'.
    |          For example, you cannot download and sell them as part of a CD library

i don't know which files are related to this license because seems unspecified