Best answer by Airgetlam
I think most iHeart radio stations have made a change so that they can't be played via TuneIn anymore. My guess would be that they really want people to use their own tools, so that they can not lose the advertising dollars to TuneIn. Have you tried using the iHeart radio part of the Sonos controller app, rather than the TuneIn part?
If so, it might be helpful to Sonos if you were to both submit a system diagnostic, and post the number here, for the folks at Sonos to look at, as well as indicating which particular radio station you're having issues with.