As far as pure software development (not using at that point integrated custom hardware) as an idea to form, I haven't seen anything in over twenty years that deserves a patent. That includes audio, video and other compression systems that build on algorithms that are over two decades old. Those systems that tightly work with specialized hardware are a bit of a different story.
As far as any system that is defined only in software, and capable of running on a generic computer, with generic input/output devices of today are not, imho, patentable. Anything that involves an obvious idea implemented in an obvious way is not patentable, of which the incredible majority of software is. Again, exception to interfacing specialized hardware...
