Hi there,
The requirements are quite clear and straightforward to implement. Just like magnet links, e.g magnet:?xt=, with torrents the application will register for a certain kind of protocol, e.g protocol_name:... on the system. When such link is clicked in the browser the program will be initialized and process the audio and video with ffmpeg to create a combined video output with sound. I'm not sure if you want a GUI to track the process/conversion or just a console app doing the job would suffice. Also, where should be output file saved to? Maybe that might be a settings on the program with GUI? As a side note, please let me know if you want anything else on such GUI.
I'll walk you through the signing procedure, no worries in this regard.
Per your request, I'll use c# to implement the program. I am planning to target .net 4.5. If you require a GUI I'll use winforms. This'll be ready for testing in 2 weeks, thanks.