You'll have to take all 19 hidden radios (Some in plain sight, but ignore that), to their hidden spots. For example, the first radio in the room you begin in, if taken to the button in the next room, will start playing Morse Code. This will unlock 1/19 of them.
This site has a description of where every radio is, and where to bring them (Too much to list all here in one answer!):
http://grumpygamers.com/2010/03/02/portal-transmission-received-achievement-walkthrough/