For a CPU cooler, if you want the most effective cooler which is silent, it's probably this one... CR-100A IcePipe 100W Fanless CPU Cooler
Problem is, it's absolutely huge and it's quite expensive if I remember correctly... You do get other passive CPU coolers, not too expensive, but only good if you're not overclocking or putting it through top heavy use, and top heavy use could come with music production.
For fans, I could recommend spending at least around £15 on each fan, will get you results. I've got some Noctua's and you hardly notice they're there, Nocta also do CPU coolers, should imagine they're very effective, but a passive cooler could prove best if you want more results.
Next up, PSU, cheap ones are nasty and noisy, the more you spend, the more power you'll be able to distribute across your rig, the more power efficient it'll be, the safer your components will be and the longer you won't need to buy another for.
You can get up to a platinum power rating (infact, I think you can get higher now, but will stick with platinum because I'm not sure).
Anything above silver gets pricey but could be worth it, I have an XFX PSU and that's a very stable PSU, was never really bothered about the sound so much, but it does produce some, it's not silent. You'll need to do quite some research about dead quiet PSU's...
I purchased a passive cooling GPU, it's a Gigabyte something or another, can't remember the model name from the top of my head, it has the ATI / AMD Radeon HD 5750 chipset though. Its cooling is surprisingly effective, however, a good airflow within the case is a must, if you don't have that, passive cooling will be useless (that goes for the CPU cooler too)...
Hard drives can also be quite noisy, clicking away, if you have a slave drive (say for backup and such), you'll certainly know about that when it starts up as you try to access it. You'll know even more when it powers down, especially older hard drives.
So if you can, SSD's are the way forward, if not, just run your OS's and smaller programs on the SSD's and put bigger programs and project files on a seperate HDD. At the moment, I run my OS, Cubase and other smaller programs from my SSD, then all of the big programs and Cubase projects from a WD Velociraptor (great drive by the way).
Next up, CD drive, you can get quiet ones, just have to shop for them, if you have any old CD drive, it will kick and scream, everytime I start up my PC, I usually have a DVD in there and it's just the noisiest thing in there, sounds like your PC has its own little turbine, so I need to invest in a quiet drive, I'd recommend shopping around for one if you need it...
Last thing I can think of... Casing, which I do believe is what you originally asked about, the more you spend on a case, the better built it will be and will provide natural sound proofing because the case will tend to be thicker or of a certain material. However, you will always hear something, I have seen purpose built cases for audio work. I can't give you any names, but they do exist but they can be rather expensive to my understanding. Even if they cost the manufacturer £5 to build, they'll charge you for function and that's that, it's just something you have to accept.
You may even be able to get a rackmounted case, could look dead cool and sit flush with other rack gear you may have.
Sorry I can't be of much help in the case department, I got a Zalman Z11 Plus, it looks nice, really well built and a great price. They supply most of the fans for you infact, but it's noisy and I haven't got the money to replace all of the fans just at the moment.
Quickly overviewing your rig specs though, very, very nice build... However, I think the RAM could be a big overkill, I have 8GB and have no troubles, but then again, you may do a lot more application heavy work to me, so 16GB should be fine. You can always upgrade to 32GB at a later point (although I have heard leaving RAM slots empty can bottleneck performance in some way or another, I've got two empty slots, still no troubles with RAM).
Picked a good CPU, Seasonic is a very good brand, motherboard is excellent, thunderbolt may be a good investment, no one can tell yet, they're coming out with new technology before the previous can actually kick off...
With the GPU, I would consider looking into a passive cooled one, could make a big difference.
If you're running SSD's only, you won't really need any cooling around the HDD bays, the PSU creates a bit of airflow within the case, so I would probably suggest 2 - 3 case fans. One to intake (probably best to locate at the rear of the case), two to push out the air (one on the side and dependent on what case you get, one on the top or elsewhere).
One more thing, you also do get acoustic treatment for your case believe it or not, amazing I know... I think Alienware do some among other companies, I'm sure you could just make it yourself if you had the right material, could be worth looking in to. I doubt it'll make a huge, huge difference, may knock a db or two off though.
Apologies for the essay and unnecessary details, it's my day off of work and I'm bored
Hope this helps!