Best front-end language for desktop multimedia?

I’m contemplating developing a new bit of software. Requirements are: 1) Runs on Linux and Windows, 2) Can easily support development and responsive playing of audio and video, animation is a plus, 3) must either be a solid back-end language for file access, shell commands and LAN networking, or easily integrate with a language that is good at these things, 4) Must be able to run as a full-screen application.

My initial thought is a Flash front-end communicating with a Java back-end, but I don’t really know the particulars of how this would work, tips would be helpful. I’m shying away from .NET because I don’t trust Microsoft not to kill MONO in the blink of an eye, and Java because Java GUI development is a pain. Think I’m wrong, convince me otherwise. Think I’m missing something? Enlighten me.