I had the same problem with “Simon says …” when I tried to play mp3-files on echos when I followed the instructions from Mark Watt Tech https://www.youtube.com/watch?v=ZJlH6k9PY4I
Finally, I could fix it by correcting the source-statement of the mp3-file.
It seems that the source needs to be like this (watch the different quotation marks):
src='https://abcdef.ui.nabu.casa/local/mp3/Alarm.mp3'/>