[DanHulton]

IANAL either, but I'd say it's pretty transformative. It goes from binary code and assets you can interact with to a video with commentary.

Song mashups already go from songs to similar-sounding songs and those are fine. Again, IANAL, but if mashups are okay, it makes sense to me that Let's Play videos should fall under the same - if not more generous - rules.

[skymt]

But legally mashups aren't okay. Bridgeport Music, Inc. v. Dimension Films was a sucessful suit over a two-second sample. The court decided that unlicensed use of even this brief riff was infringement. Mashups are tolerated because of their place in musical culture, but if a copyright owner decided to sue they'd have an easy case. (Though maybe not from a PR perspective.) (I'm also not a lawyer.)