A warning: This post is quite long, and has a fair bit of detail. I wanted to put this info out there, but haven’t spent a huge amount of time tidying it up or making it digestible. Ask questions in the comments if you’d like more info or clarification.
I've been trying to fiddle about with a 6 channel audio track, trying to get a movie soundtrack without the dialogue. The basic principle is that most of the dialogue is in the centre channel, so inverting left and adding that to right strips the dialogue, but keeps the music (and sound effects). I was using audacity, but the size of the files meant it brought my computer to its knees (because of hard drive access and a stupidly large paging file).
After some searching I found vox, which is a command line tool. It's pretty much like piping text from one tool to another, but with audio 😀 yummm!