I've composed a set of system sound purposed for Intrepid Ibex. Please cast your vote!
The set is soft and clean system sounds.
The files called "sound" followed by a number, is sounds designed to be assigned to an informational message, Warning message, question dialog or similar.
This package now includes two versions - a rhodes version (the old) and a TS-404 version.
On screenshot 2 you can see what commands the sounds are attached to. I would also suggest to use Sound7.wav for Question Dialog or perhaps even for exit/shutdown.