Sometimes, if I enter a moral choice selection screen, the diplomacy screen of a race, or spend too much time in a single planet building things, the sound of the game will bug out and stop. Clicking on ships usually resolves the issue. Still some sound cues won't play sometimes (e.g. end of turn, idle ship given a command etc). I haven't had to restart the game yet to fix it, and only once did the sound stay off for more than a few turns. I suspect this has something to do with over-usage of RAM and CPU resources (although I have a 6-core 3.2 GHz CPU and 16GB DDR3 RAM)