my question is : did videohive authors can use unlicenced soundtracks in their project previews ? ( like “killling dubstep” from youtube and of course with name of hwo created it in description)
Everything in your preview must be licensed for commercial use or your own work, otherwise you would be infringing other people’s copyrights.
For using those dubstep tracks from YouTube, I don’t think that an attribution is enough to go without a permission from its owner/s. (you had better double-check this with the Help Team).
Also I highly recommend reading this helpful article at the Help Center: What Assets Can I Use In My Items?.