When you assign a button to control the "play" button for Live Tracks in Sunday Keys, you'll want to make sure the Type is Absolute and then it will work as expected.
Sunday Keys Version 2
Using the Tonic Pad Player
Getting Started with Sunday Keys for MainStage
Troubleshooting Sunday Keys for Ableton
What Gear do I need?
Troubleshooting Sunday Guitar
My Song-Specific Patch is Different from the Demo
Customizing Your Song-Specific Patch
My instrument/controller isn't connecting