To quote the pop-up help in the Midi window in the mac version, "To prevent any volume changes within BIAB, uncheck this box." Well, that doesn't work. I think it might be a bug. I uncheck it... then when I load up my songs, each song still changes the volume to whatever it was saved with.
Anyways, stupid work around? I used a keyboard macro to re-save all my files with a new setting. Almost 200 songs. pretty annoying.