Mp3 audio files compress songs by removing inaudible sounds, allowing for portable music storage on devices like mp3 players. However, transferring music to these devices can be challenging for some. CD sales declined between 2008 and 2015 as digital music grew in popularity due to CDs being breakable and digital music being cheaper and easier to access on devices. Various audio formats are used for different broadcast products including terrestrial radio, internet radio, and in-store music systems as well as recorded content like podcasts.