No 5.1 isn't a plug in. It's a format of recording. You;ve probably seen it in home theatre setups. The format got its start in the movie theatre. You know how you can go to see a movie and you hear most of the sounds coming from in front of you, but then everyone in a while you'll hear something coming directly from your left or your right? That's 5.1. Its not a plug in or an effect, it's actually mixing with the intent of using more than 2 speakers.
Regular audio is 2 channel. Right and Left. 5.1 is six channels. So you have your right and left, plus you have a center channel, 2 surround channels, and your subwoofer channel. The .1 is the subwoofer channel becuase it actually has a max frequency range that you can put through to it, becuase its supposed to be just for the bass, so its not a full 22KHZ signal, hence the 5.1 instead of 6, get it?
There's no way to take a stereo audio recording and just use a plugin to make it 5.1. Well there are some ways to do that, but they're all booty. You actually have to make the decisions when your mixing, what do you want to go to what channels? And then when your done you have to mix it down to a special format. You can't just mix it down to a stereo wav and then convert that to an mp3. You wont be able to play your 5.1 mix on an mp3 player.
It's pretty ineteresting next level shit. Not many producers **** with it. You'll have to do your own research to figure out how it's done, and probably the final product you'll end up with it a DVD. I looked into a long time ago but never actually produced something in the format.
Here's a couple links for you.
http://en.wikipedia.org/wiki/5.1_surround_sound
http://www.idmforums.com/showthread.php?t=6826
http://emusician.com/tutorials/5-1_mix/
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You also might want to look into Binaural Recording:
http://www.gearslutz.com/board/so-m...78936-binaural-recording-mixing-speakers.html