I do believe that there are a lot more Window programmers than Linux; same can be said for the MAC too. The world is a Windows world. I disagree with you about knowing what to remove from Windows to make it secure, if it was that easy those SysAdmin in large organizations would have done so, but how compatible is the system and more important how stable is it after key components have been removed? I’d never accept a System that runs a custom version of Windows.
As for those manufacturers that create DVR cards and those whom just tweak them up a bit for their own box is what I’m talking about. That describes a large segment of the Video Surveillance industry. Someone could make money cranking about DVR cards with a good API into Linux(which would not be that expensive and would be relatively easy to do) and boom a money making machine is born.
Just my thought.
As for audio Rory, I don’t know if there are not any, just I have not found one. Nonetheless DVR_Expert_Australia has a great point on how to handle audio if you were serious about using a Linux based box and it had no audio.