CC-BY 3.0 license and author not respond question

CC-BY 3.0 license and author not respond question

Hello all,

I'm building an ios game and I want to use some sound effects from opengameart.org. Actually I found 2 sound effects under CC-BY 3.0 license so basicly that mean I nead a permistion from the author but what happen if the author not respond to my email, Shall I just use it and mention his name in the credit game?