Steve,
I just did a search on Youtube and here's a clip that might help. It's a great video.
https://www.youtube.com/watch?v=GY_KufYoon8At around 4:30 onwards, the guy talks about setting sounds on individual channels. Even though he's using Cubase, the principle for BIAB would be the same since it's necessary to assign sounds to midi channels so that BIAB can play them.
Regards,
Noel