On these websites you can download music or sound effects for your Unreal Engine 4 trailers. Remember that the free license is valid on Social Networks or Websites due to the mandatory attribution.
1 - Best Free Audio: https://bestfreeaudio.com/
2 - Mixkit: https://mixkit.co/free-stock-music/
If you know more websites that offer free music or sound effects, you can post it!!!