Sound randomly missing from game for the past 2 weeks or so...
League of Legends: Auto Attack animation sound doesn't play sometimes.
Crysis 3: Bullet sound of the mounted MLG on the mission "Only Human", doesn't play, even though it did not so long ago.
Got this a few months ago on old PC in Mount & Blade and some others, problem was cables from speakers to PC were in bad positions, switched them, all was okay.