Can you use your computer to make a radio transmitter?
The audio output doesn't operate at high enough frequencies to generate radio signals. But the VGA output does. If you take direct control of the VGA hardware, or modulate the pixel data at correct intervals, you can generate RF signals. My favourite example is Fabrice Bellard (author of FFmpeg) making a DVB-T transmitter from a VGA card: http://bellard.org/dvbt/

Depending on what you want to achieve, you might get some results if you make an induction loop, such as the ones used in hearing aids, and connect that to your PC's audio output.  

There's also an AM transmitter example using VGA output: http://www.erikyyy.de/tempest/