How does ambient sounds work?
Right now it doesn't seem to work (Github Issue). As an alternative you can try playing sounds with scripting, like play_sound("sounds/door.wav");. It’s not the same but worth having a look at. For more information to scripts see the Scripting Reference on the old SuperTux Wiki.
If AmbientSound works with a future SuperTux version, I will update this answer or you visit the SuperTux Forum with latest news and more helping people than on this Q&A.