I can reproduce with Space Blaster (please always follow the bug report guidelines though) but it looks like a Safari bug. We use the Page Visibility API to detect when the page goes in to the background and pause all audio. You can see it works if you switch tab in Safari - the audio stops correctly. However if you press the Home button Safari does not tell the page it's become hidden so it's none the wiser and keeps playing the audio.
I don't have an iOS 8 beta to hand right now to test on, but hopefully it's fixed there... if not I'll file a bug with Apple.