It depends on whether it is a class or your own individual workout. If it is a class it depends on the type of class, and even within types of class on the type of students.
I know there was an IDEA article on the topic of music with exercise a couple of issues ago. It sounds like music can be very helpful, but what you choose and how you use it is as personal and as varied as any other part of one’s exercise routine.
When I teach senior cardio I use big band, or show tunes. When I teach senior yoga I sometimes turn the music off altogether as it is easier for them to hear. When I teach a vinyasa style class the music tempo is faster than if I do a relaxation class.
For myself I like to walk my dog and listen to the world around me, or if I am on a cardio machine I like to read. When I lift I do not want music, as I want to be present fully to the experience. If I am on cardio, and if I really want to push though I have a mix tape for that.