Headphones create a disconnect between student to student and student to teacher. Yes, kids can hear you, but they don't listen to you that well. The sound in the headphones is much more interesting and immediate. They tune you out to a large degree. It is also rather impractical to ask everyone to pause their program and take off their headphones so you can talk with them.
Good teachers like to discuss things 'on the fly' as they occur.
Headphones get in the way of this. Competing computer speakers can be distracting and annoying. A lot has changed. The proper room size and set-up are the two most important issues you can address. I've heard from and seen quite a few computer labs that are set up wrong or in too small a space.