You know the god awful buzzing sound so many people reported on DS9 that was effecting 7.1 surround sound headsets? Well,, it's back.. and this time on Bajor. only unlike DS9 where it was just in the Promenade it's EVERYWHERE. Including in the training missions...... outside.. inside.. it just changes how loud and how low / high it is.
Okay, well obviously 7.1 has issues in STO. Perhaps you can work around this issue by switching from 7.1 to Stereo or Mono. It's entirely possible you won't be able to use your 7.1 sound until the DEV's have fixed this issue and yes, it's a documented issue.