Hello, thanks a lot for your quick answer!
OK, I’ll not plug the MF audio output to the audio in of the Z anymore…
I don’t have the line module which is a bit expensive I think (at least for what I need to do), and the splitter looks nice but I do not understand what it does, and why the OP-Z is able to detect audio coming from a headphone’s mic but not direct audio.
There’s certainly a technical reason behind this ^^.
So it seems that sampling directly via USB is possible, do you mean that I need to record the MF note I want to sample on my computer, then play it while sampling with the USB port of the OP-Z connected to my computer?
That’s not straightforward as I’ll need a computer every time but that sounds pretty easy.
I’m going to try this!
For sequencing, I tried different settings on the MF, I’ll try to find your post (EDIT : Found it!
How to connect OP-Z to MicroFreak? - #2 by docshermsticks).
Thanks again 
Guillaume