If had my Sonata for almost a fortnight now, and I thought I'd share a few observations I've had about it with regard to my previous setup.
I originally used an old AOpen HX-08 full tower case, and had been disappointed with the poor cooling the case offered, as it had no exhaust fan to remove CPU/Gfx card heat (Gf4Ti 4200 fitted with a Zalman ZM80). The exhaust fans were instead situated above the PSU, which meant they were next to useless: all the hot air had to be expelled through the PSU. I had my Cuda IV no-vibed, and used a Zalman Flower on my Athlon TBird 1.4
I originally changed the stock AOpen PSU for a 300W Q-Technology quiet replacement, about a year ago now. Due to the aforementioned problem of the PSU having to exhaust all the air from the case, the temp-controlled fan had to speed up to compensate, resulting in increased noise (still alot quieter than the stock PSU though). The case still got quite hot nevertheless, and I'd frequently see CPU temps of 65C and case temps of about 30C. So it was time for a case change.
I finally got my Sonata after waiting for about a fortnight (I really do wish retailers wouldn't advertise stock until they actually have it) and swapped my gear over to the stock Sonata configuration.
I switched on, and found that it was considerably louder than my previous setup. It did do a far better job of cooling though - the PSU fan was faster, louder but kept the PSU cooler at around 30C, the case temps dropped by about 10C to 20C, and the CPU temps dropped to around 49C due to the excellent job of the (rather loud) 120mm case fan.
I also found that the HD mounts didn't manage to decouple the HD enough to stop vibrations travelling through the case when seeking. So I put the Cuda back in the No Vibes cradle and put it in the remaining 5.25" bay. This did lead to the HD temps being raised by about 5C, but that's nothing to worry about. I'll have no choice but to use the drive mounts if I add a drive, but I'm confident that the newer Samsungs will be quiet enough for this. My ageing 80G Cuda was just too loud to be hushed by the grommets.
Despite the excellent cooling job provided by the Antec PSU fan, I decided to swap back my Q-Technology PSU as it was much quieter. After realising I couldn't remove the top and right part of the case cover, I carefully managed to slide the PSU out over my Flower cooler, after removing the fan assembly. It was a shame to remove the stock PSU, as it was very cool and the fan shifted a lot of air for the resulting noise. There were plenty of nice touches about the PSU too which smacked of quality, like the 3 pin molex used to monitor the PSU fan RPM, and the PSU-governed case fan connectors, which all made it a little harder to swap back my Q PSU. I'm sure it would be possible to mod the Antec PSU for quieter performance, but rather than muck around risking a shock I thought it would be safer to swap out the whole PSU.
I didn't cover the 'Antec' holes on the side of the PC, as apart from the neglible noise emanating from them, they seem to help keeping the PSU cooler and therefore quieter.
The 120mm case fan, while doing a great job at keeping the case temps low, was far too loud. I didn't want to get cutting at the grill either, so I connected it up to the 5v line of a spare molex from my Q-Technology instead to quieten/slow it down. The rubber fan mounts do a great job of isolating structure-borne vibration.
There. As quiet as my previous system yet much cooler, compact, and better looking. But I'd also just about neglected every feature on it that was supposed to keep it quiet! I suspect Antec wanted to err on the side of caution when balancing noise with cooling ability, but still managing to acheive a quieter setup than your average stock PC. I'm sure you could do a fair amount of modding to get it quieter, but I think I'm already quite far along the path of diminishing returns, and don't feel it will be worth the effort TBO. I might get a few Panaflows to replace the 92mm Zalman and the 120mm case fan though, but I suspect this will be too costly (in the UK anyway!) to make it worthwhile. I have been thinking about a Silenx PSU though...
Sorry if I've missed any crucial points, and if anyone has any questions I'll do my best to answer them.