Psst 2.1.1 for Mac can be downloaded from our website for free. The program lies within System Tools, more precisely System Optimization. This free software for Mac OS X was originally created by Satsumac Software.
From the developer:
Psst is a faceless application that lets you adjust/mute the volume of your Mac's startup chime. Double-Click " Psst !" in the Finder to adjust the volume settings. Important: The method Psst ! uses to change the volume does not work on every model! What it actually does: It simply adjusts the volume settings before shutting down and after logging in.
You may want to check out more software for Mac, such as Robot School. Programming For Kids, which might be related to Psst.