How To Duck Audio in Logic Pro X
This can be done using sidechain compression in Logic. 
Create a new track and insert a sound (eg Kick drum) on every part you want the sound to duck, then set the output to no output. 
Go to the track you want to duck and add a compressor and then set the side chain input to the track you created in the step above. 
Tweak the threshold, attack and release settings to get your desired effect.