So it's only recently I've noticed that Dota 2 couriers actually have some sound effects like Mighty Boar's oinks when he spawns. I loved courier sounds in WC3 Dota, especially the penguin one. But in order to hear that I had to turn my in-game volume way up high. On my "default" volume settings every sound is ok apart from the couriers being way to quiet. I suggest you rescale the volume pitch of the couriers so they can be heard on the lower volume settings.