In short:
1. Yes, a warrior can be upgraded to have sound after the fact. It is not an inexpensive upgrade, and it is best done by someone familiar with the process. It can be sent back to SF for the upgrade, which would preserve the warranty, or to some of the talented smiths out there who do this sort of thing, which would not preserve the warranty.
2. I think you will find that the new 12w+ LEDs fill out an infinity edge blade pretty darned well. Mainly, because the individual dies are closer together than the regular 12w, and they are brighter. However, if you wanted to try a blade from another vendor, a standard 1" blade should work just fine.
3. The quick connect allows you to swap out the LED unit in your saber without having to cut wires, and solder them back together, thereby violating your warranty. They are a set of connectors on the wires leading to the LED unit, that snap together, and unsnap to allow for the easy swap. You just have to ensure that any extra LED units you buy, that you include the quick connect option on those as well. Typically adds a little to the price of the saber, and to the price of the individual LED units, but well worth it.
Guess that wasn't so short after all.