Google is in the process of making Audio Ads available to U.S. AdWords advertisers. The platform, which auctions off remnant inventory on radio stations, has been in beta since December, and was expanded in March.
Barry Schwartz has several screenshots of the interface at Search Engine Roundtable.
Audio Ads competitor Bid4Spots currently serves the U.S. radio market, and announced plans to expand its radio ad auction to the U.K. last week.