On Cake 6, I've been recording and mixing midi tracks until my project suits me, then turning it into audio by (1) inserting an audio track into Cake, (2) switching off the USB to Main on my Behringer mixer, then (3) pressing Record on Cake. This lets my Ketron module's patches play into the audio track which I then export for use in gigs. Today, for the first time, I experimented by inserting some Cakewalk Sound Center tracks (softsynth included with Cake 6). The tracks play fine when recording midi but when I switch the mixer for the audio recording as mentioned above, they do not make it to the audio track. Only the Ketron tracks sound. I assume there is some setting that needs made within Cake. 

There is no need to record audio from a soft synth but it is possible to render the audio if desired by either freezing or bouncing the tracks. No need to get external hardware involved. it all happens in the DAW. This is not necessary however; soft synth audio will export without rendering beforehand.
 

I use the freeze track function. It's easy to undo and make changes if needed. In addition, it removes that synth from the processing loop and reduces the load on the processor.

Well hot diggidy dog. You guys are smarter 'n red suspenders. It worked!