I must say my initial judgment of easiness was not correct. I have read the instruction here
(perhaps you have read it and it looked easy, then today saw your reply and thought it may not be not easy and not safe , so I decided to try it myself.
Anyway I had to use the instruction and improvise a little bit, but I change the sound and I can from now on change the startup sound within a minute, because once you change some settings you don't have to do it again if you decided you get tired of the sound. BTW I had to manipulate a dll, but with no risk as the original is there and I can replace the original any moment.
Still if want to do it:
1. Change the ownership of the dll mentioned to Administrator.
2. Change the permissions so that you can create and change the dll (this one is not mentioned there I had to find it myself)
3. You need Resource Hacker (read and if you wanted download from the link). You should right click it and select Run as Administrator.
4. Replace the WAVE file mentioned in the link, fill in WAVE, 5051,1033 in 3 open windows. Click Save under file. The Reshacker makes a backup of the original dll (imageres_original.dll) you can any moment rename to imageres.dll and it restores the original windows sound.
All above steps are clearly mentioned in the reference link. If you still decided to do it and needed I can tell you step by step the number 2.
Read the first instruction (7 step, then 4 steps and then comment from kekoa plus the 2 step mentioned here.