Using free UE4 assets (Soul: City and others) as promo material

I want to sell audio I’ve created on the marketplace and through my own website. I want to use the visuals from some of Unreal’s assets that have been released for free - Infinity Blade, Soul and Paragon. I WILL NOT be selling these assets, simply using the visuals for a short video in which I will clearly state that audio is the only thing the customer is receiving, and I own all legal rights to the audio having created it myself. Selling audio by itself is incredibly hard, people need visuals to understand the potential of how audio can fit into their project.

Am I allowed to use Unreal’s intellectual property in purely promotional ways?

As long as you use it with Unreal Engine 4 ONLY.
If you would like to read about selling assets on the UE4 Marketplace, look here: Marketplace Guidelines - Unreal Engine