technically even 1 second of a song could be copyright infringement. there's not an acceptable level.
It doesn't need to be someone reporting stuff. We had discussed this months ago with samhabib.
youtube has an content ID system. If the copyright owners make an application and send reference material, they cross reference it to uploaded material and remove it automatically. as you can see below soundcloud also has the same system in place and they would automatically take down anything that had received a take down notice.
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What is Content ID?
YouTube's state-of-the-art technologies let rights owners:
Identify user-uploaded videos comprised entirely OR partially of their content, and
Choose, in advance, what they want to happen when those videos are found. Make money from them. Get stats on them. Or block them from YouTube altogether.
It's up to you.
How does Content ID work?
Rights holders deliver YouTube reference files (audio-only or video) of content they own, metadata describing that content, and policies on what they want YouTube to do when we find a match. As you can see soundcloud also has the same system (see info below). I would say that most probably by now Sony / Web sheriff made an content removal request.
We compare videos uploaded to YouTube against those reference files.
Our technology automatically identifies your content and applies your preferred policy: monetize, track, or block.
http://www.youtube.com/t/contentid
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Soundcloud
Starting in the last few weeks we’ve turned on an automatic content identification system, similar to those used on other major media sharing sites. The system is used primarily for identifying audio that rightsholders have requested to be taken off SoundCloud. This is good news because it makes it easier for artists, labels and other content owners to control how the content they’ve created is available.
http://blog.soundcloud.com/2011/01/05/q-and-a-content-identification-system/