Just tested with a few other aircraft where i could find mp-sound configuration. These also cause freezes in the same way as 777:
bell-407
hughes500
ja37-viggen
So i guess it is not a "777 / mp sound" issue, but a "any aircraft with correct mp sound config / mp sound" issue. I think there should be looked into the sound processing logic of fg. Have looked at sourceforge, and there is something done in 2015 regarding sound coming from different orientations. Probably that was between 3.4 and rc3.6.
Kind regards, Vincent