as far as I could see and hear on Youtube, this MX-K2 is using a piezo 'buzzer' inside.
Such piezo buzzer producing nothing like a clean 750Hz (or nearby frequency) sound.
Use a simple microphone and a freeware audio editor, like Audacity and record the sound.
You will be surprised - yes it's a 'buzzer' and not a tone generator. Morse Expert is using
a complex software to analyze and decode - but it can not handle such 'buzzing' sounds.
Hope this helps to understand ;-)
73's de OE1MWW
Wolfgang