To make a file sound like it's over the radio, what you want to do is put it through a band pass filter.
Basically, that just chops off the sound below a certain value, and above a certain value (which is what a real radio does, as it doesn't transmit across all sound bands
)
Hopefully your sound editor (sorry, I'm not familiar with Goldwave) will have a 'quick filters' section. If you can find one that does radio effect (or even telephone), you should be right