Insulation, Radiant and Vapor Barriers - Sound insulation in walls

Hi,

I'd like to sound proof some of the rooms in my house. I don't need to go crazy on this but right now it feels like the walls are made from paper.

I'd really like to avoid attaching soundproof materials to the walls and then installing second layer of drywall.

I've heard it a few times that there is a way to inject foam (or some other material) through small holes in the wall but I've never got the chance to get all the information like what type of material should be injected, what machine do I need, is there a company that does this.

Injecting foam into the existing walls will be the best solution for me.

Does anyone know if this is possible?

Any information, web link or even a company that does it will be appreciated.

Thanks
Peter

FWIW I had all my interior walls insulated with bat insulation, works very well. Not sure how sound proof you need but you cant hear someone using the bathroom while in the next room w/ a common wall.
s/t

Here's an article I found on a quick google search. (http://www.onlinetips.org/soundproofing-interior-walls)

To boil it down, they suggest adding more layers of sheetrock with silicon between layers.

--Peter