Hi Paul,
Congratulations on your new Tundra!!....and welcome to TS.
Wow!....That's pretty cool..Borla's installed as an option. You mentioned purchasing it in Georgia. I think that's SE Toyota. Sounds like a Distributor installed option.
I have the Borla catback also, which I ordered through Summit and had installed locally. I kept the original muffler and tailpipe, just in case I decide to reinstall later on.
Regarding your question about muffling the Borla to a quieter tone. I don't think you can, as the muffler itself is designed to emit a throaty exhaust note. You don't have to replace the tailpipes, but you could always replace just the muffler itself with a stock Tundra or aftermarket muffler. Definitely hold on to the Borla muffler though, if you decide to replace it, because it's made of stainless steel and will last forever.
As for the exhaust note, I like the loud throaty exhaust note you get from a V8, equipped with catbacks like Borla, Flows or TRD.
Good luck on getting it modified to your satisfaction. You have lots of options and it's not an expensive fix.