Well...
when turned it on in the bios, and the Realtek drivers are installed for the internal audio, there's no noise when BIAB is set to use that output instead of my audio interface. Hooray I thought. I appeared to have a solution. However... the mini-jack in the back of the mb has only one channel working and the front header mini-jack works but emits a very low volume (but still audible) beep about once every second, when the signal is dormant. Can't make the beep go away, so I uninstalled the Realtek drivers and shut the mb audio off in the BIOS. This said, there may be some problem with the connectors and the onboard audio in general which is preventing me from using that as a workaround.
Again, that wouldn't explain why BIAB won't play cleanly - and Realband will - through all three external audio interfaces I've tried, would it? I'm assuming that while they're companion products (BIAB and RealBand), how they each talk to drivers may be quite different.

Bruce Hook - Soprano, Alto and Tenor Saxes
BIAB 2023 UltraPak | Intel i7 | Mackie ProFX10v3 Audio Interface | 1TB SSD | Reaper DAW
| Akai EWI 5000 | M-Audio Oxygen Pro 49