Ever since I installed Avast Antivirus Proffesional 2010, I am having no sound in-game, while I do have sound in the lobby.
Maybe any way I can add League as a trusted program with avast?

Hey ProR2D2,

Have you check your Windows audio settings while you are in-game? Windows has separate sliders for each program, and this causes there to be separate sliders for the lobby and in-game.

If that doesn't work, and your in-game audio settings are all correct in LoL, you could try the repair function on the patcher. Check those things out and let us know if you have any luck.