Now the cool part is, you can add subfolders inside of the Sound Effects folder to place your own sounds into! I created a new subfolder titled "iMovie 2 Sounds" and then dropped in all of those great old sounds. I then added appropriately titled subfolders for my other sound file collections. I noticed no ill effects from doing this, excluding a couple of extra seconds at boot...
[Editor's note: This works exactly as described, and is quite cool! As I'm the paranoid sort, I actually copied the old sounds into the new folder instead of moving them. Sure enough, re-launch iMovie and there's the new folder with the "classic" sounds available.]

