It depends on the game, if you are on live poker or
online poker.
Online poker: I think that you can do whatever you feel that take out the best of you whether is music, tv, adult stuff. I usually play some metal bands like messhuggah or something that makes me pay extreme attention to the statistics of other players or just on seeing what I am doing preflop, flop and after flop in order to improve my style of game.
In real poker: I really love hearing the noise of the chips rolling down the table and what everybody is saying. I would not like to be the guy that takes out the earbud asking ”how much did he raised?” that's very awkward and I cringe every time I hear that one on my table. So be careful with the volume!
Another thing that I found interesting on a mental book about poker is that hearing soft, relaxing music like birds singing or whatever yogic thing. It is not a good thing to hear if you want to be 100% alert on what you are doing but it is really good to hear when you are on the break so you can unstress you mental activity and be more focused after the break.