Does anybody know the rules about putting copyrighted music in a video online; I want to make a video of my blender works thus far, but would i get in trouble by having a song on it from like, postal service or something?
It’s illegal (unless the song is no longer copyrighted, which I doubt) but it wouldn’t be the worst offense concerning copyright you find on the internet.
As long as you don’t make any profit and don’t get well-known, I doubt anyone would sue you.
It won’t be a problem as long as you give the artist credit and don’t try to sell your work for a profit.
Well it would be still illegal even if you gived the artist(s) credit, since doesn’t it require some sort of permission from the artist(s)? Even though i don’t think someone will sue you for putting one or two tracks into your works.
Why not make this a thread for discussing copyrights generally? Then we could discuss a very unclear thing, called “abandonware” 
The definitive site for US Copyright law is http://www.loc.gov/copyright
It is certainly probably the case that, if you are using the music as a “snippet” for non-commercial purposes, and you make a reasonable effort to include proper copyright-notices in the trailer of your work (perhaps including “used without permission”), you’ll be okay. Simply because (a) you acknowledged the owner’s copyright, and (b) you didn’t actually harm anything. That (although “INAL”) would most-likely qualify as “fair use.”
In any such material, I would suggest that you “be careful to play it safe.” Use only a snippet of the copyrighted work; specifically identify the work in the trailer of your piece; and include a copyright notice covering the entire piece under your own name. The latter is important because thereby you make it clear to all that you did not intend to put the work “in the public domain.”
For anything substantial, of course the best advice is “go ahead, ask permission!” 
Better safe than sorry.