Or is it that the baseline noise of average PCs has gone down so much lately that silencing them isn't so interesting to a wide audience?
This would be my guess. Also manufacturers are paying more attention to the amount of noise their products put out. A few years ago you would be lucky to see a mention of noise in any component review, now it's pretty common (though in a very unscientific manner).
In a way this whole site and it's success stemmed from the terrible state of affairs some years back. You really had to put a lot of effort to get a silent computer, and even when buying the parts recommended in this site you would have to change the fans, undervolt them and undervolt your processor to keep things under control.
Now it's possible to get something as silent as the above with off the shelf components, and you even have choice. Naturally this will show up here as slower activity, and less need for users to actually register and post.
I got an iMac a few years ago, and it was surprisingly silent off the bat. I still have it which is one big reason I don't spend as much time here. When I was getting a htpc I came to have a look at what people where recommending here, and got enough information to not have to post any questions. Same thing for now when I'm considering setting up a NAS (ok, I've always thought SPCR should be covering NAS devices in their reviews too), I'm finding good enough info using the search to not have to post. Only reason I logged in really was to answer this post.
I don't mean to say this service is no longer needed, it definitely is. There is just such a wealth of information already here that iit might in itself slow down the accumulation.
ps. do some audio testing for NAS devices, or teach smallnetbuilder how to improve their reviews from that aspect