Flex 6400+Maestro configuring for WSJT-X...further questions

You do have CAT ports and Audio selections correct.
So at this point;  two more things to check;

A.  On Maestro or SmartSDR, make sure that for Audio menu, DAX is selected, and highlighted.
If DAX is not selected, you will see this same issue.

B.  What Windows version and build# are you running?  Recent Win 10 updates are causing the
same issue;  DAX channel shows streaming level, but no audio getting through.
The symptoms are that when you click the windows 10 speaker icon in the bottom right
notification bar, and select SOUNDS, the two TABs on that dialog box will show PLAYBACK 
devices and RECORD devices.  The corresponding DAX devices on each TAB will have names
and sub-names that no longer match, i.e.,  Flex Radio DAX RX3 might also say DAX IQ RX3.
You can search here for other threads that show the corruption, and what to do.
But that usually ends up requiring a trouble ticket and a teamviewer remote session
with support.

Cheers,

Neal