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i think you're trying with standalone versions of ZynAddSubFx, either the JACK client or plain ALSA PCM one.

first, when dealing with Qtractor that is, you shall stay with the JACK option as the ALSA one will need exclusive access to the soundcard device which most probably is needed and blocked by the JACK infra-structure, of course.

second, when using a standalone JACK client, you must be aware that it only works as an external soft-synth: you shall then connect Qtractor MIDI output to ZynAddSubFx MIDI input and its JACK audio outputs in turn to whatever your soundcard device/speaker ports are, usually system:playback_N (N=1, 2, ...).

third goes like a suggestion: try getting stuff from the KXStudio repositories, if you haven't already. I'm sure you can find there the most recent builds for ZynAddSubFx, especially in the LV2 plugin format, within the so called DISTRHO plugin collection.

check it out for yourself, you won't regret ;)
cheers