Thanks to everybody for their help!
When all files in STK are 'free' in the FSF/Debian-sense (requires no limitations on commercial use), the game will be easier to spread. There are a few sounds that do not fit FSF/Debian-licensing requirements.
What sounds are problematic?
* crash [now using replacement: http://www.freesound.org/people/qubodup/sounds/147660/]
* energy_bar_full [now using replacement from krank]
* grab_collectable [now using this but seems to not be a perfect match]
* pre_start_race [Now using http://opengameart.org/content/race-start-beeps]
* start_race [Now using http://opengameart.org/content/race-start-beeps]
* skid [perfect loop made]
* strike [qubodup contacted two Freesound artists for permission to use under bysa3 on 2011-07-19; got one positive reply on same day, now using this]
* liberated bowling_roll [qubodup contacted source files owner via PM on freesound.org on 2011-07-18; got positive reply on same day]
by Marianne Gagnon (Auria), based on "PaintCan", "StoneSpinLong" and "PaintCanDropHardAndRoll" by daveincamas from Freesound
What to do?
- Contact authors of sound and request use of their sound under FSF/Debian-compatible license (only possible with the freesound sound in this case)
- Find freely licensed replacement sounds
- Find non-free replacement sounds and ask owners for permission to use under FSF/Debian-compatible license (this worked for me 90% of the time on Freesound) or
- Create sounds from scratch or by modifying freely licensed sounds
Where to get freely licensed replacement/source files?
- Wikimedia Commons Sounds
- Other open source game projects via FreeArtSearch (licenses need to be checked)
Why do we need to replace these sounds?
- Tintagel apparently just found sound files on the internet and assumed they are public domain. We must assume they may not be used commercially.
- SoundSnap license prohibits resale, which is a restriction on commercial use not tolerable by FSF/Debian guidelines.
- Creative-Common Sampling Plus doesn't allow use in advertising, which is a restriction on commercial use not tolerable by FSF/Debian guidelines.
Everyone who enjoys fiddling with sound effects: please join!