Pandora.com, the online music service that has only experimented with audio ads in the past, is incorporating regular commercial breaks into its service.

15-second ads will air just once every two hours, says Tim Westergren, Pandora founder (via MediaBuyerPlanner and AdWeek). The ads are reinforced with graphical images within the Pandora web interface.

Initial advertisers include American Idol, Bose and McDonald’s.

To peddle ad space, Pandora hired two sales executives from the traditional radio world, according to ClickZ.

Pandora experimented with ads in early 2007, with a 9-second McDonald’s ad between songs when a user changed stations; each user heard the ad once per day. But when the company asked for a reaction, most reported being upset with the addition of ads, so Pandora ceased running them, writes CNET.

Users disgruntled about audio ads this time around can register for premium membership — $36 per year. Premium members suffer no ad messages.

Pandora hosts 21 million registered users and 2 million regular users per day. Advertising in music streams, on its website and via the iPhone are its main source of income.