You can use /sound, but that requires the user to have the song already.
You would have to write both a client (to connect and listen to the stream) and a server (to send the stream) if you want to do this. It's not an easy thing to do.
Please also be aware that it's illegal to broadcast copyrighted content without permission.
If you really want to do some sort of "radio", then you're better off doing it outside of mIRC and just giving people the URL to it. There are various streaming media apps available. Don't forget what hixxy said about copyrighted material, though.