Skullworx is the only all-inclusive system I know of and installs in about 5 minutes. Quality is good and it's certainly convenient. That said, it's just a basic analog sound system that you could do yourself with a bit of time.
The TCI is a great piece of kit and is pretty easy to install and doesn't take much effort esp. if you get Stomper's hovis with speakers installed.
Of all the sound systems I've heard though, nothing but nothing beats the RomFx. They are *very* pricey given the exchange rate but if you want the best...
For sizing, my garrison mate actually has his RomFx and battery boxes (double A no less) 100% self-contained in the helmet, so it can certainly be done.
Actually I sold my helmet that had vox and so am currently vox-less actually until I can get a new RomFx. I think he's working on a bluetooth system, so am saving my pennies for magic to happen next year.