Cool game, but the demo is to short for you to consider buying it in my opinion
I agree. Here's a method to reset the demo. It may seem like a lot of steps, but it doesn't cost that much time:
1) Go to the directory you installed Steam in, then go to steamapps/common/audiosurf;
2) Rename that directory to 'audiosurf_old' or something. Don't close the Explorer-window, we'll need it later on;
3) In Steam, right-click on the 'AudioSurf Demo'-entry, and select 'Delete local content', then select 'Delete' in the window that pops up;
4) Go to the AudioSurf page in Steam, click 'Play the demo', and select 'Install demo';
5) Immediately pause the download in the 'My Games'-list, and close Steam;
6) Go back to the 'common'-directory. Delete the 'audiosurf'-directory, and rename your 'audiosurf_old'-directory back to 'audiosurf';
7) Start Steam up. Resume the download of the AudioSurf demo. It'll quickly increase to 100 %.
8) When it's finished, start the demo up. Voilą: 4 songs remaining in demo!