Pd is a real-time graphical programming environment for audio and graphical
processing.  It resembles the Max/MSP system but is much simpler and more
portable; also Pd has two features not (yet) showing up in Max/MSP: first,
via Mark Dank's GEM package, Pd can be used for simultaneous computer
animation and computer audio.  Second, an experimental facility is provided
for defining and accessing data structures.

The MIDI support implemented in this port is derived from Raphael Graf's
patches for a sndio backend on the OpenBSD audio/portmidi port, applied to an
embedded PortMIDI library, enhanced to detect a variable number of MIDI (umidi)
devices:
http://openbsd-archive.7691.n7.nabble.com/audio-portmidi-input-td363848.html
https://marc.info/?l=openbsd-ports&m=155221816900336&w=2

Unofficial web site: http://puredata.org/