C:\Program Files\Steam\steamapps\common\audiosurf\engine\textures
Plus a few other files for skyboxes and road colour and such. They're easy enough to edit, so people do. Even I did; For a little while I'd turned the emphasis rings into little vortices.
Most of the community effort is here, though people do make their own threads.
http://www.audio-surf.com/forum/inde...ic,2498.0.html
Anyway, screwing with the wrong file tends to make the game go back to the alpha code, which had the above joke in it. No idea what it means.
Two things I like are
for rings
and
for score particles.