Headphones were a set of two speakers that fit over one's ears to allow one to listen closely to a transmitted message.
In a 2374 Hirogen simulation of Nazi Germany, an SS Radio Corporal wore headphones. (VOY: "The Killing Game")
In 2399, Doctor Agnes Jurati wore headphones while she was listening to Kasseelian opera outside the Daystrom Institute in Okinawa. (PIC: "The End is the Beginning")