I would like to automate the following to work with AviDemux. I can get it to produce very good results that play well on my home theater system, but doing this, one file at a time, is a pain.

I start with an MP4 file, and use the following settings:

Video: I keep at Copy.

Audio: I switch to MP3, then choose Stereo from the Mixer drop-down found when I click on the Filters button.

Format: I switch to MKV.

At this point, I save to MKV, using the same filename with an MKV extension.

All in all, it's pretty straightforward, with only one setting changed, but I've not seen any examples of the use of the Stereo filter.

Thanks for any help!