Google advertises its Audio Ads program, as Marshall M. found out. Give it a try by searching US Google for audio ads.
You'll see an AdWords box reading "Audio Ads by Google - Now it's simple & easy to target customers on the radio." The landing page for this says:
Interested in advertising your business on the radio?
Google Audio Ads allows you to broadcast your message to thousands of listeners and brings simplicity, efficiency, and accountability to radio advertising. Here's a brief overview:
- Select from hundreds of radio stations across the U.S. to run your audio ads
- Target your customers by station type, demographics, and time of day
- View online reports that show you when and where your ad played
- Listen to real-time recordings of your ads as they were played on the radio