Just drop .ogg-files here, they will be used in the game. Which track
will be used is decided pseudo-randomly with the level name as the seed.
That means that the tracks will be chosen randomly, but they will always
be the same for every level. Unless you add new tracks.

sfml2 seems to have a problem with normalized oggs. Use a normalization
factor of 0.90, e.g.:

$ sox -v 0.90 input.ogg output.ogg
