There are many, many, many, many (did we mention "many"?) synthesizers out there in the world that you can control using MIDI note and CC controls.
You may already have a favorite software synth, and chances are it'll work with the Trellis M4. In case you don't have one already picked out, here are some good ones to try.
Nearly any software synth (or DAW (digital audio workstation) you find will allow you to use USB MIDI to control the notes, pitch bend, and CC input. Here are some good ones to try:
Chrome Web Browser
free open source
- AudioKit Synth One (iPad only)