Miller Puckette's book has a whole chapter devoted to the general
topic of control and audio streams:
http://crca.ucsd.edu/~msp/techniques/latest/book-html/node40.html
Note that it would be a misunderstanding of Pd if you view it just as
a software implementation of a modular synthesizer. In fact, Pd's
predecessor (IRCAM Max) didn't have audio capabilities at all.