Didn't see this one mentioned anywhere...
How difficult would it be to make modifications to the ships or use new ones? Probably because I want to make the skysphere a picture of stars and planets, I would absolutely turn Mono into, say, the Millennium Falcon and cruise down a course in that ship, or maybe the Firefly Serenity.
That would be badass.
My understanding is that if you have a legal copy of Quest3D you can make assets that will work with Audiosurf.
Without that, however, you can only switch the character ships about (there's a couple of unused ship models in there as well you can switch in).
Quest3D is obscenely expensive (cheapest version is 1249 up to the most expensive at 9999), considering how buggy it is, I'm surprised the make any sales when compared to something like Unity (free for PC, Mac and web).
Anyway, Quest3D comes with an exporter for I think it's 3DS max (or whatever it's called these days) which will allow you to use them in Audiosurf.