I'd like to have a setup option, where I can toggle audio between 5.1 and stereo. I would have two separate audio tracks (ac3 for 5.1 and wav or something for stereo).

Sony DVD architect (the non-pro version) doesn't support 5.1 sound at all.
I started searching: can it be done in multiAVCHD ? combine it with tsMuxer ?

thanks.