Web apps in iOS8 that play audio no longer play audio in the background. Once you close Safari audio stops playing.
Also and previous to iOS8 audio playing in Safari would keep the iDevice in a semi-wake stage. This is no longer the case and audio stops playing as the devices sleeps.
We're (speakerblast) hoping this is a bug Apple will fix. I'm sure GrooveShark is too, as I would bet their HTML5 app is a popular destination for their iOS user base.