Some technical disciplines use "safe" as a transitive verb meaning "to make it safe", either from damage or from damaging other things.

I've seen it most often in rocketry contexts - you "safe the rocket" in an aborted launch scenario so it won't go off accidentally, etc.

  "It is a good idea to add a safety release valve to your launcher. Sometimes it is important to abort a launch and safe the rocket after it has been pressurised." http://www.aircommandrockets.com/rocket_launcher.htm (Google "safe the rocket" for occasional other examples.)  

So the PRE-SAFE sound means the sound is to "safe your ears" before the crash, making them safe from damage.