Gameplay: No sound once the game starts

Hello I have a problem with sound.There is no issue with my computer the sounds works fine. It also works while navigating trough the client and during champion select, yet once the game starts either in SR or ARAM the is no sound or ambient music. no sound from champions, items, shop, etc. I thought it was my computer, but is not. Help? I don't know how to fix it since when i go the option menu and check the sound everything is fine.

Thank you