ZZAMBONI Some of these are nice sounds. One problem is that many of the files are 4 seconds long, even when the sound itself takes one second or less, with three seconds of silence at the end. I think these should be cut off. (Version 1.0)
The best thing to do is to create a Sounds folder in /Library (the main library) and put these files into it. This way they will be available for all the applications which can use system sounds, e.g. 3-2-1 timer, in addition to being available under System Preferences>Sound>Sound Effects. (Version 1.0)
RORO01 This tip is no longer relevant as the developer took it into account and introduced a graphical installer in version 1.1 which installs the sounds in /Library/Sounds. So they are in fact system sounds. (Version 1.1)