Is there an "audio" button on your DVD remote? You can change between different audio options by pushing this when the workout is playing.
On the DVD itself, when you choose to play the program, you are automatically taken to a menu screen that offers you the option of "Normla Mix" "Vocals only" or "Music louder mix'." Highlight the option you want by using the 'up/down' controls on your remote, and press "enter."