Is it possible to use the headphones that have a little mic on them? I believe most of the ones sold at Five-Below now have that. Then you could send the mic audio with a slight delay (the memory buffer in the sound card might be enough on its own) to the output audio so that they hear themselves and how loud they are? Most of those mics are set for the frequency range of a human's voice and filtered (cheaply) to just catch the closest voice.