Just downloaded PD 0.51.1 and running it on Windows10. Got sound working right out of the box, but it has stopped working without me changing any of the preferences. I first_sequencer.pd I've attached my simple patch here. Basically it worked until I added a new message (62) to go into the makenote object. Since then, I've fiddled with different combinations of IO, MIDI, etc. to no avail. I don't know if it's something weird with GS Wavetable Synth of MIDI Mapper, or if I'm missing something super obvious.
I did the audio/MIDI test patch, which worked, though the tone at 60 was originally a sine wave, and now sounds like noise (though I can change the pitch of the noise by changing the number in the message). Maybe that's a clue? But I get a nice middle C piano note when I click the MIDI box.
Totally stumped right at the beginning!