In the Halo series, there are several tracks with murmurs in them. These seem illegible, until you reverse and speed them up. Then they become as clear as day, and they tell the story of the forerunners and how they perished by the flood's hand. Gravemind keeps repeating this.
There are a few of these. Some of them in Halo 2, like the first video, and some more in Halo 3.
Below is a video of reversed Black tower. After that is a reversed video of floodgate.
It's very interesting that Bungie would put messages like this into the Halo games over the years.