Band in a Box does not generate any Music XML at this point in time.
...
We send the file to the Sinsy site and that is where the Music XML generation actually comes from.
--Mac
From my testing, I'm not sure that is correct. The very first thing that happens when you click on the Vocal Synth button in BIAB is that a file, Sound.XML is created in the folder /bb/Data/SRequest
This file appears immediately, well before the Sinsy site completes or maybe even begins its work. It seems to happen so fast that I doubt it is a file being returned by the Sinsy site, but I could be wrong.
Once Sinsy has finished, another file called sound.WAV will appear in that same folder. You can see the difference in the time stamps is a minute or two. There are two other files, SRequest Input and Output that show the same time stamp difference.
EDIT. To test this, I disconnected my Internet connection. BIAB gave me an error message that it could not process the SRequest, but it did create the Sound.XML file. Thus, BIAB is generating Music XML.