In garageband ducking audio was a one-click operation. However I cannot find how to preform the same action 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.