Mellotron Micro 

The problem was the key range. The sounds got the original range from the F below the middle octave setting to a F above the middle octave setting. So only the middle octave setting would play on all keys. If you use the transpose function, the bottom 5 keys are without any sound. And the top octave transpose setting would make the top 7 keys silence.
And after booting it won't play any sound at all until you moved the octave transpose switch. Strange.