Standard copyright applies. There is fair use, but just putting someone their music in your 1 minute long video is a no-no. There are licenses like Creative Commons. For example, Karoliina Salminen remixed a video of Peter Schneider intro speech at Maemo Summit, and added her own audio work to support the visual remix. If license is unclear, talk to author(s). OTOH, we have no control over Youtube and copyright infringement occurring there...