OK, for me a mixer is a thing you use when you want to put inputs from various sources into something - recorder, computer, transmitter etc - at the same time.
What I want to find out about is the opposite of that but googling "Thingy to let you run four sets of headphones, each with their own volume control, from one headphone socket" probably won't get me very far!
Please, what do you call it? I bet someone out there will know.

Eddie