I think that by eschewing video and audio entirely, you're missing out. As apple has demonstrated, animations and transitions are important to the user experience and user understanding. I would definitely allow screenboard creators to import or upload video and audio clips. Perhaps this could be a feature for advanced/paying users.
Audio is pretty important too. Sometimes, explaining things via voice or with sounds are more efficient than writing. You can convey emotion and emphasis through voice that don't come through via a limited text palette. The web audio api part of HTML5 is getting better and I would investigate using this. Again, for advanced/paying users.
But seriously, thanks for building this. I've wanted something like this for a long time.