Combining online audio streaming with on-air radio transmission 
is a key activity for the Xchange streaming media network.
Once you have your streaming media rig setup you could, 
if you had the inclination, hook it up to an FM transmitter.
Potentially, depending on the range of your transmitter, you 
could take audio streams from all over the world and broadcast them 
in your local area. 
See Radio 90 in Banff, Canada, which does exactly this
See 'How To Be A Community Radio Station' 
for information on how to get hold of a transmitter.