You probably already fixed this, but I ran into the same thing. Its looping because you are telling it to Do Once after it players once, and then the hit fires off again. Youd think Hit fires just one time when it touches but, it continuously fires off as objects roll or slide against things too, so thats why its looping is because Hit continues to fire off.
What I did was make a Sphere Collision component in my actor, and then mark it to overlap my world or other solid object types, and make sure it has overlap events turned on. Then I just adjusted the radius to be a generous but small size to loosely fit the object that the sound goes with, and then I use End Overlap on that Sphere Collision to reset the Do Once playing the sound. So Hit goes to Do Once, play the sound, and then End Overlap resets.