I ran a few minutes of the mission I'm on using the fastdebug exe and here is the log:https://fsnebula.org/log/5e6fc9a2cb0d3321b1c9861b
Of particular note to me is this section, lines 529-538:
MISSION LOAD: 'Md-04.fs2'
Hmmm... Extension passed to mission_load...
Reassigning player to squadron Galactic Terran Intelligence
Someone passed an extension to bm_load for file 'GTI.pcx'
Someone passed an extension to bm_load for file 'gta.pcx'
Someone passed an extension to bm_load for file 'pvn.pcx'
Starting mission message count : 208
Ending mission message count : 214
Current soundtrack set to -1 in event_music_reset_choices
Current soundtrack set to -1 in event_music_set_soundtrack
Don't know much about the inner workings of FSOpen or the FSPort, but wouldn't a -1 in either of these turn off the soundtrack? Idle speculation