Do you plan to make money off of your mashups?
If so, I hope you have deep pockets, else, good luck with the legal battles for fair use... unfortunately this is no longer the late 80's/early 90's when copyright was so far out of touch with technology, it was sample heaven!!
Nowdays, I seriously doubt anyone (I should say Record Company) would pursue you for mashups being used in audiosurf. However, it could be argued that if you're offering your mashups to me, and others on here (at no cost of course) you are then perpetuating music theft, or... partial theft as it may be!
I am a firm believer in 'fair use', if I wanted to
make money using samples that were obviously created by someone else, and pertain to someone else's livelyhood, I would seek legal advice, and discuss the use with the relevant parties. As many artists do if they were releasing a Top40 single!!
In your position, I would quite happily create my music for FREE distribution, and cross the legal bridge when, or even if it ever comes.
Basically 'they' don't want you to profit from 'their' stuff, unless there is something in it for them!!
Which for all intensive purposes is fairly understandable. Even if it is artistically limiting!!
In my opinion, an artist should never be limited because of Copyright, hindered prehaps, limited no!!
Oherwise, art, and in this case, specifically music, will not progress.
I am not a Lawyer. This is only my opinion. I do not condone piracy. I do condone 'fair use'
Here is a very interesting video on the Amen break! It deals with the above-mentioned issues. (Yes, you have heard it
)
http://www.youtube.com/watch?v=5SaFTm2bcacDJ Evil
Sinister Tones