Thanks for the info. I will probably not change default alarm sound many times, so it is enough to do it once from the terminal for me. It worked fine to move a new alarm sound I wanted to /usr/share/sounds/freedesktop/stereo, and rename it alarm-clock-elapsed.ogg.
Anyone looking for a more calm alarm sound, I can recommend one of the default alarm sounds in android called “Argon”. I sent it over from my android phone running a free android OS, but I am apparently not allowed to upload .ogg-files here.