I've also encountered this problem in every game since Beta 5, although for me the sounds never play instead of 50% of the time. Usually starts around turn 70 but sometimes later. When this starts there will also be sudden bursts of sound whenever I open menus like the shipyard or the designer.
I've also encountered these bugs when playing with my friend. Another bug we've run into is that when we reload the game everything that my friend explored is unexplored again, but he can suddenly see everything that I've explored.
For anyone who hasn't or doesn't want to play through the campaign there is a great summary/timeline of the story available here: https://www.galciv3.com/databanks/our-story It covers what happened before the campaign started (History of the Drengins and Arceans, the Stargates, how the Terrans took their place among the stars), what happens during the campaigns and what situation is at the end of GalCiv2. <