Just mime the act of putting on headphones.
Adjust your handshape to reflect the size and shape of the headphones.
Here I'm putting on some "medium size" headphones using "bent 3" handshapes.
Ear buds would be shown using "G" handshapes.
Earmuffs would be shown using "bent 5" handshapes.
American Sign Language University ASL resources by Lifeprint.com Dr. William Vicars