It's already against the rules.
I was just on a flight to Barbados last Saturday with a family of 6 occupying 4 seats in F (2 lap children) across the isle. One of the kids was having a temper tantrum (for about the 50th time) and dad put a video player in the seat back and put the volume at about 11 (sorry for the Spinal Tap reference) so the kid would behave.
Took all of about 30 seconds for the FA to come back and let them know about the ear bud policy. They were NOT happy about it.