Saynomore:
That's the only example I found of Reaper MIDI track sending informations to Arduino...
I don't think you need examples of Reaper sending MIDI info to Arduino.
Just
- figure out how you connect Reaper to external MIDI devices through USB. Probably just set your dedicated track in Reaper to some midi channel no other instrument or device uses.
- figure out how to read incoming data from the USB to Arduino as MIDI data.
An alternative to Arduino reading MIDI data through its USB is to build a separate midi connection for your Arduino, say with DIN contacts (old school MIDI standard). Then you need a USB to MIDI converter.