Listening music is fine. However, keep in mind that the more focus on muscle movement the better. Just like Arnold said, you want to focus on the muscles as much as possible, get inside the muscle, put all the attention there. Arnold would not even answer a phone call to avoid getting distracted while training. There are people that go to the gym and they are totally distracted while lifting, well they won't get far. Or when I go to the park running, I see some people completely assorted into their problems, with their head looking down instead of focusing on the beauty of nature. What's the point on going to the park if their mind is not there. The concept of being present ! This could be a bit off-topic, but I still wanted to mention it since it could be of help.