A lot of stations have gone digital with systems like Scott Studios (
http://www.scottstudios.com/). They use MP4, if I remember correctly. I worked for a network that switched to Scott, and it's awesome. The stations I worked for used Minidisk and CDs before... I think they had an array of something like 96 players, all controlled from a crazy minidisk/cdplayer thing. It was a little before my time.