This is an element of the way many engines handle sounds, not so much a feature. When an attack is cast the "cast" sound is played. When the attack lands the "attack landed" sound plays. When you cast a power there is only one caster. When the attack lands it may hit several targets at once. Meaning that more than one "attack landing" sound is played. Most games have a set time between Cast and Land. So each "attack landed" sound plays at the same time X amount of time after cast.

When you stack multiple sound waves on top of each other you get a louder sound. Because each "Attack landed" sound effect is the exact same wave that starts at the exact same time they add up into a much louder sound. 3 people clapping together versus 3 people just clapping out of sync. Basically this effect is just happening because of the convenience and simplicity of not calculating time of flight for the power that was cast and playing the audio only when the power actually reaches the target. Some engines also don't vary the volume of near and far attacks that are landing at the same time. They play X effect at Y volume from both speakers if it's on the screen and they might be slightly more nuanced if the sound is to your left and right.

I hope this helped explain what is going on. I've been told I'm very bad at explaining things this week. Well they didn't say I was bad at explaining things they just told me that my ideas wouldn't work before executing them a few minutes later.

Curious how UE handles sound be default. That might be a good research topic that probably wouldn't take too long. But I gotta go to the BMV tomorrow and wait in line. TTFN.