sound deadening: probably dynamat? I haven't looked into sound deadening stuff in quite a while. It'd likely be the same basic stuff as long as sound deadening materials have been around: doors, maybe parts under the carpet, liftgate, etc. This car is much quieter than my 2002, so the ride seems quiet as hell for me. Should be noted that I, too, am in a desert climate, and I've taken my 18 Sport Hatch on non-paved roads. lol

Sway bar: cheapest used to be the 20.5mm FK8 CTR rear sway bar. That is, untill Honda got wind that all of us were buying up the FK8 rear sway bar for like $70, so they upped the list price to the $200 range. I managed to hop on that bandwagon before the price hike. Some people don't like the added "oversteer" from the CTR bar, so instead, they upgrade to the Si 18mm rear sway bar. Both are a direct drop in using stock end links and cross-compatible with all 10thgen coupe/sedan/hatches. Otherwise, other companies out there: Whiteline 22mm adjustable rear sway bar (MSRP $160), Progress 20.6mm (~$170), Eibach 22mm (~$140). I'm sure there are others out there, but I can't think of them at the moment. Word of caution: it's a huge pain in the *** to tetris the bar in. Swapping out the bar should've been a 15 minute swap had it not been for how "fun" it was to put in. I think it took me a solid half hour to get it in place to where it'd bolt in.

As for your whistling AC, couldn't tell you. I haven't heard anything like that in mine

Warranty: Take it to your dealership, DEMONSTRATE THE ISSUE so they know exactly what your problem is, without question.
They will determine if it's either normal for the car or it's a problem.