(With thanks to lebeth, I still went and did this with pictures.

)
1. Open up Steam, and go to your My Games window.
Right-click on Audiosurf and select
Properties.

2. Select the
Local Files tab in the window that appears.

3. Click on the
Verify integrity of game cache button.

4. A nice window will pop up showing the progress of the verification. When it reaches 100%, the window will disappear. Try loading Audiosurf again, hopefully your problem will be fixed. ^^
