Selecting workout music to listen to is about as personal as it comes; everyone has different preferences for what they enjoy. The important thing to remember is that the music should not be the focus of the workout, the workout is the focus of the workout. Do not get caught up in the music and lose one's concentration on the task at hand. The majority of people listen to music throughout their workout as background noise but there are other uses for music during a workout session like timing.
Using music for timing
Those that walk or run on a regular basis on a treadmill (it is not recommended to listen to music while walking or running outdoors in high traffic or user heavy public places as it can drown out traffic noise and jeopardize one's safety as well as others) are especially prone to listening to music to break up the monotony of the activity. Some will even make a specific playlist with a warm-up, main run, and cool down track for the length of the exercise session.
This technique can also be applied for lifting weights as well. Rather than using the classic set/rep lifting method. It is possible to perform circuit training with music where you lift one muscle group for part of the song, then superset it with another muscle group for the other part of the song. Or one can do a series of different exercises during the length of a song. This can be particularly effective with ab exercises where one perform do a wide variety of exercises to target various aspects of the muscle group.
In many studio classes music is the cornerstone of activities. From spinning sessions to boot camp classes, music is used to set the pace of the workouts and motivate individuals to push beyond their normal limits.
As mentioned above, it is not advised to have music playing if one enjoys going outside for their cardio. Loud music can make it difficult to hear traffic noise or other people out for their cardio programs. Other types of people that may be out for a cardio workout may include walkers, joggers, bikers, rollerbladers, all needing their own space and traveling at their own pace. Communication is key to keeping public paths and trails safe for everyone.
These safety issues are not just specific to those participating in outdoor cardio workouts; they also apply to those individuals at the gym. Some people listen to music so loud that they are unable to hear other individuals attempting to catch their attention. This can be particularly frustrating and potentially unsafe in the free weight area as it is possible for dumbbells to go astray.