I got a reply from support that helped!

"You'll also want to select the SD-50 as your audio driver. In Band-in-a-Box go to Opt | Preferences | Audio and change your driver type to ASIO. Then select the SD-50 as your ASIO device, check the box for ASIO Always On and hit OK. That should ensure that you're sending all the audio to the SD-50 in addition to all your MIDI data."

After I did this, I found I still had to do one more thing:

At first, I only got the Real Instrument & Drum sounds, so I went to the Driver Wizard Piano Test, which also was also silent. After checking to "Use my external MIDI module ...." and selecting the SD-50 did I get the Piano Test to work and after that I returned to playing a song and everything was there!

Brgds,
Tonnie