Hi Hamai, yes, you're right. We had a close look at the TOS of all the sources we're currently supporting and offering the possibility to use only the sound of a YouTube video is one of the things we're not allowed to do.
Hi Steren, yes, we're currently working on a pure HTML5 version of Weavly.
We’ve made the decision to start with a Flash version first as embedding an application like the Weavly player in the Facebook stream is currently only possible by using Flash - and we wanted to be present on Facebook from day one. But getting the HTML5 version finished is our next target and we should be able to publish it within the coming weeks.
One problem I have: With a slow connection it doesn't play smoothly but stops after every vine.
Dislosure: I know the people behind weavly and slept on their couch last year.