Copy and paste this into your description :
class CfgRadio
{
class Alpharadio1{name = "";sound[] = {"Alpharadio1.ogg", db, 1.0};title = $STRM_Alpharadio1;};
};
Then, in your stringtable :
STRM_Alpharadio1,"Alpha 1 here... reading you loud and clear",
(Make sure you've got BOTH commas).
Once you've done this, RELOAD your mission. One useful thing to know is that the description.ext file is only taken into account when you LOAD the mission : you thus won't see the changes if you don't reload the mission first.
All this should work.