I have the Antec Sonata case too. I love it, I sometimes can't tell if it's on or not because I unplugged the front panel LEDs except for HDD access. (tended to shine right in my eyes when playing games in the dark.)
You'll want a fan that moves a decent amount of air for the 120mm slot. It is the only thing that moves air past the hard drives, and it does it by making a negative pressure in the case to suck air in through the case hole in front of the hard drive bays. So you need something strong enough to suck air through the "Antec" shaped case holes and the filtered hole in front of that bay. (so I would say use a Vantec Stealth, except I don't think that moves enough air.)
I'm currently using an Antec 120mm fan that I think came with the case but I can't remember... The fan is 120mm x 120mm x 25mm, but other than that I don't know its specs. I do know that with that fan, my 92mm processor fan, the power supply that came with the case, a HighTech Radeon 9800 Pro, and a mobo that doesn't need a chipset fan, I literally sit down at this sometimes after windows has put the monitor in power save and think it's off. (reach for the switch, see the HDD light blip, think "oh wait... it's on...", and smile.) And my HDD's stay nice and cool as long as the case cover is on.
No, actually it's linoleum floor and wooden and drywall walls. It's a big room though. the computer is on the desk.
Did you plug that case fan into the "Fan Only" connector from the power supply? If it gets full 12 volts it is indeed loud, but plugged into the Fan Only connector it still moves plenty of air and it's completely quiet.