No Sound with USB Stick

If I don't insert a USB stick, I get a sound.
I use the Raspberry Pi 2, NI Komplete Kontrol S88 MK2 and the standard audio output.
If I plug in a USB stick, I get no sound.
On the stick there is only a folder called “1 Mellotron MkII Flute” and the samples in it.
Can I somehow see what the Raspberry Pi is doing during this?

I have now figured it out: I have to start the first sample set with 0.
=> 0 Mellotron MkII Flute