I personally think listening to music is a good idea. I feel it is unlikely to damage your concentration and may even serve as a mood enhancer for those of you prone to tilt. I listen to music often when I play, and simply find my awareness of it diminishes during important hands, but comes to the fore to keep me sane whilst card dead.
I admit, I do occasionally watch films and things whilst playing, although I notice such actions receive a lot of bad press. I never play more than 2 tables when watching a film though and I prefer it in situations where you are likely to be doing a lot of folding anyway; for example MTTs or even DoN sngs.
The truth is, a film you have not seen before generally requires a large amount of your concentration. If you are trying to follow a complex plot, either it is just not going to happen, or your card playing is going to suffer. It is much easier to watch a film you know well, because then it becomes like the music. Your awareness of the movie can ebb and flow since you can just watch scenes when you are card-dead, but not have to worry about missing plotlines when you have playable hands.
Overall, I think nothing is objectively bad to do whilst playing, you just have to understand realistically the toll it is taking on your concentration. I'd say the effect of something such as music is minimal, even positive.
Occasionally I have attempted to read PS articles and other stuff while playing, but it is difficult to truly play your A-game and get the best out of the article at the same time.
I also like to have sex while playing 24 tables. Honest.
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