Audio for some games stopped working...

I don't know why but some games that I play have no audio. I go in the settings of the games and its not the game itself. I think its my computer and I'm not sure if its some settings that got put on some how. One thing im confused about is, it's not all the games, some do have sound but its not the ones I play the most. What should I do?

Games I play are, Without sound: StarCraft 2, Diablo 3, Team Fortress 2, Garry's Mod (or simplified my Valve and Blizzard games)
With sound: Terraria, Realm Of The Mad God,