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Putting snd_rebuildaudiocache into the console makes the game crash. Any subsequent attempts to launch the game makes it crash at the title.
This is literally just Portal 1 but with missing textures in the windows. Absolutely no beta content was here.
Never Mind. It turns out that the way to install this is different from the way you described.
I got it to work by putting the folder in C:\Program Files (x86)\Steam\SteamApps\common\Portal\portal\custom rather than putting it in C:\Program Files (x86)\Steam\SteamApps\sourcemods
For some reason it crashes while loading the title screen.