Is it acceptable for professors to listen to music using headphones in the workplace when expecting student contact? In my institution, I see a lot of research students and staff members listen to music using headphones in the workplace. It is a matter of personal preference, and I cannot see a problem in that while it doesn't interfere with communications at work.

Is that behaviour acceptable for a professor during a time in which s/he expects students to approach her/him?
Yes, of course. As long as they turn it down so that they can talk over it when the student comes by, why would it be unacceptable to have music on while they are waiting? If they are listening on headphones while waiting, it would be preferable if they could see the student approaching, but there's nothing unprofessional about it even if they don't. The student can knock on the door, wall, floor, desk, etc. in order to get their attention.