I guess you're are right you can't be sure the player is on or what the volume is. But the buds alone must mask a bit. Also if all they want is the handsfree they could leave the bud out of one ear.
My only experience of this is waiting to get into a locked bike shed behind some guy with earbuds in, he dropped his wallet while getting his keys out. I picked it up and told him and he didn't turn round. I had to tap him on the shoulder to get his attention.