Ayant trouvé la solution, en grande partie grâce à l'excellent service technique de Motu, je partage l'info
Voici les infos de Motu, mais attention cela ne suffit pas, il faut autoriser les logiciels audio (Pro Tools et Ableton dans mon cas) dans l'onglet Microphone situé dans Confidentialité et sécurité
MOTU :
I'd recommend a full uninstall of any MOTU drivers that are currently on the system, and reinstalling using the instructions below:
Open a Finder Window
Click GO in the Menu Bar and select “Computer”
Open the Startup Disk (usually called “Macintosh HD”) and go to Library
Open “Extensions”
Delete all MOTU items
go to Applications and delete the MOTU Pro Audio System Extension
Download and install this version of the driver:
https://motu.com/en-us/download-center/download/1954
Immediately after installation, restart the computer and follow the instructions in this link:
https://motu.com/techsupport/technotes/enabling-motu-drivers-high-sierra