if you need a program to mix tracks in a session, then there's no point in even thinking of programming something like that in pd, just use any of the very good programs around.
There is a very simple and oft-overlooked point to reinventing such programmes inside of pd : it gives you a mixing tool that becomes endlessly reconfigurable, and it gives you components that are useful inside of pd patches that aren't fundamentally about mixing.
I doubt that Pd users are the kind of people who use any ordinary tools while NOT thinking : « hey, if I had this in Pd, I could be plugging [whatever~] into it, or I could plug this mixer into blahblah.pd ».
Actually I often plug Pd into Ardour (which is a DAW, and is developed, also, for mixing tracks) through jack. it's so simple. :)
so you do exactly what I said, use a specifc program for a specific task.
if you use pd+ardour together, you might be interested in my patch to
control ardour from pd through osc:
https://pure-data.svn.sourceforge.net/svnroot/pure-data/trunk/abstractions/j...,
ardourjack-gui.pd.