What radio are you talking about?

You need to make a ambient generic with the music you want. in the ambient generic you set the object that gives that sound (if im right the normal sound is something with radio_loop - filtering on radio will make you find it for sure). Then make a prop_physics with the model Radio_reference.mdl (as told above me). the prop_physics should have the same name as the object you wanted to give the sound of the ambient generic.

i dont know the exact values of the sound but it shouldnt be hard to find the settings you like.

Well, in the following topic you can download decompiles of the original maps. Simply remake the radio from those maps. testchmb_a_00 has a radio in it as thats the map you start in.

http://www.fpsbanana.com/threads/95314?14

What does it say?

How did you do it? Post so others can find out if they have the same problem.