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What about OSS? I understand that it's legacy, so maybe we can just use the portaudio implementation?

i think the merit of OSS is that it allows to build Pd with *zero* external dependencies and still have sound (and MIDI): no libalsa, no libjack.

this is probably also the most compelling reason for MMIO.

You can build portaudio with just OSS resp. MMIO support and get the same result :-)

(Not to mention that on Windows there are much better native options, like WASAPI.)