Can't say about the first one. It's probably a bit quieter than normal PC due to the thick wooden walls. The problem with that one is, that it has openings for the sound to escape from. Silencing is like the old saying about weakest link... If you leave one weak spot, then the sound gets out from there
The guy who build the last one says:"Lopputulos oli odotettu, kone on nyt todella hiljainen. Jos ikkunat sulkee ja kaikki muut laitteet sammuttaa, hyvin pieni humina kuuluu vielä. ", which translates to: "As expected, the computer is now extremely quiet. If I close the windows (not the MS one) and shut everything else down, I can hear very slight hum." So I guess if there are any background noise and you can't hear it.
The second one is a lightweight version of the third one (The second and third (and last) are based on a design by the same guy). So the second one is bit more noisy than the third one. The thing about those air channels is that there is no direct way for the sound to escape from the box. When the sound "bounces" (reflects) in those channels, it's also weakened (absorbed).