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In the last riddle, the one were you have to replay the melody, the first button of the keyboard doesn't make any sound...
the other ones sounds a note, but the first is mute...

I resolved the riddle thanks to an online solution, that given me the right sequence comprehensive that "mute note"...

Am I the only one who have this issue ?
This question / problem has been solved by qwafflerimage
it just sounds muted.
it is actually a very low bass sound which is hard to hear.
it could be a problem with the speakers.