open the Sound Recorder, but how do I make it to start recording?
That's not the kind of thing that subprocess was intended for. That kind of behavior is more akin to win32 hooks.
Subprocess can manipulate std input but not buttons on a GUI. As sneekula said, if the windows sound recorder can be fully utilized through command line processes then you'll be good to go, otherwise you'll need to figure out if that program has any win32 handles to work with.