I guess it can be done but not proberly at all ....
The player hears a sound coming from an entity as far as he is from it ( sound MUST be in the Sound0 properties filed , and started with sound=$0 action ) but if the sound is long it will keep on the same volume that it started until it's looped again ( you get me ? ) , anyway you can make your radio voice about 2-5 seconds to get a nice effect .
I might be wrong so test it .
Cheers
"I am the night visitor when everyone is sleep , and when the last light fades"
The illusionest