Despite having read this thread I'm still not getting the proper syntax to bacth mux some files under win. Having to use ffmpeg or mencoder makes no difference as long as the job gets done.

a) add a new audio stream from an external mp3 as secondary stream to an AVI
b) ignore the two AC3 streams of an AVI and replace them with two external mp3 streams (adding a +950 ms delay).

Any help getting the right syntax for any tool is appreciated.