Ask HN: Using other videos/music as footage - Copyright issue

me and friends are organizing a small TEDx conference and we're making a small promotion video for the event. Then suddenly all the copyright issues brought up when we try to use some other good existing videos on youtube as excerpts.

Then it gets me thinking. I saw a lot of youtube videos that use other videos and music (the ones where you create a photo album of your trip's pictures and insert a background music, or making an inspirational video with music etc).

My question is, is it legal to do so? Do people usually ask the owner of the video/music/pictures before using them in their videos?

The other day I submitted a video to youtube and got and email say the music I used is copyrighted by some music group. I, following other videos, carefully put a note in both the video and description of youtube. I know this wouldn't be enough. But how about other tons of videos out there?