Another issue, when you have more than one audio track, by default the upmixer upmix all of them no choice. This is not good here as in some case you want to keep some track like they are, so could you please add an option to select which one you want to keep as is, which one you want to upmix?
This would be in addition to my first request, this should not be difficult to do here, you have all the code already, just enable to have these options in the GUI.
