I heard also some time ago that Windows Phone 8 would a great leap forward for html5 apps and it wasn't true at all, then I heard later that Windows Phone 8.1 would be the best platform for html5 and it wasn't true at all again. So I'm not excited at all about spartan. Every, and I mean EVERY platform that claimed great things for html5 developers has been a constant source of disappointment in the last three years.
Audio bug is simple to explain. Streaming is not consistent, so your music sometimes stops and there's no way to avoid it and If you play more than one sound at the same time time your app has a great chance to crash.
AdDuplex is not a monetization service, it's a promotional tool. AFAIK there's no way to monetize a windows phone 8.1 app made with C2 through ads.