Rich,

If I may, let me try to explain. .SGU and .MGU are BIAB files. The BIAB program can play those files with output either as midi or as audio (or even as a mixture of the two). Whether the BIAB output is midi or audio is determined by whether you are using "Real" instruments or "Midi" instruments within the BIAB program. Those green instrument headers tells you that currently BIAB is outputting as audio, thus there is no midi data being sent to your external synth (or anywhere else for that matter).

To change the BIAB output to all midi, you need to select a midi style. Open the style picker, and under the "type" selector, pick "midi only". Then select one of those styles. Now all those instrument headers should be yellow.

Oh, I think the BIAB program default is to substitute "real" drums, so the drums may be green. If so, click on the "Real Drums" icon, and deselect "automatically substitute real drums for midi". Now the drums header should be yellow as well. If you now generate and play your song, all the output will be midi (BIAB will not output any audio, only midi). The sound you hear now comes from whatever midi synth is selected as Kent outlined above.

Bruce

Macbook Pro 2018
2.6 GHz 6-core Intel Core i7, 16 GB RAM
OS Monterey
BIAB for Mac 2022