Is there a device, e.g. a USB device or slot card, that can broadcast the audio from my computer to my AM/FM radio? I have looked around and can’t find anything that will do that.
An FM modulator will do it. Anything more powerful is typically illegal. The downside is the FM modulator is the sound is terrible, and depending on where you live you might not find a clear signal. Best bet is to get a good set of speakers for the computer. Don't waste your money on the modulator.
Melody - visit the iPod/MP3 section of your nearest Best Buy or Target. As Timmy points out, all you need is a transmitter. You can find audio transmitters that will plug into your earphone port of your PC and transmit the audio on a particular radio frequency, and you can listen to it with your radio. This is a pretty cool thing to do if your computer is in a part of the house where you can't hear it, but you want to listen to your downloaded music elsewhere in the house - so you're thinking along the right lines!
All you need is an FM transmitter. Run a search on eBay and you'll get a whole list of car/mobile versions but sift through enough and you'll find one with normal audio. I just ran a search for FM transmitter pc.
Is this what you are looking for?
Use a FM modalator