Sorry, this will be a terrible bug report.
It basically happened at random.
I started the game (729) and it works fine, then some sessions later it was muted. Restarted and everything was ok again. Later, it got muted one more time, and again, just restarting it fixed.
When it got muted, it was muted all the way from launching the game and its start screen. Sound options were at 100%.
Maybe it is related with the audio changes and antivirus? Because in this new version (and the last one too), AVG stopped the first launch of the game to scan it. Following sessions were ignored, probably the av marked it as safe.