I hope you've find the problem by this time, but if you haven't, I've a solution and if someone else is searching for it, he might find my reply useful.
I don't know exactly why but the sounds that you've in your Radio folder will be played, but extremely quietly, so if you want to hear them, you'll have to amplify him drastically.
For example :
class welcome
{ name = "welcome"; // Name for mission editor sound[] = {\sound\welcome.ogg, +60db, 1.0}; title = ""; // Text for radio message };
PS: Still, be careful not to explode your ears with something like + 180db !