Thank for your reply Abacab.
When I insert TX16Wx in Sonar by selecting "MIDI source" and "All synth audio outputs" -- which is how I do for all multi-out Vstis -- I also get 8 audio tracks (4 stereo pairs + 4 mono) plus one MIDI track.
Unfortunately, when I select the outputs from within Tx16 -- just like I do with other VST hosts I use Tx16wx with and with which I'm not experiencing this issue -- either from the left pane or by overriding this setting and choosing an individual output from Tx16's keyboard mapper, I only get to hear whatever's assigned to the primary stereo out (S1+2).
See the attached screenshot


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Tx 16's developer, to whom I reported the issue commented that :
If the meters are silent (and you did the routing right - which I assume, since you did the same in reaper), then the host did not connect the pins. TX only shows output that is actually rendered to host. Any idea about what I could try?
Thanks again,
J.