From Halopedia, the Halo wiki
Reversed messages refer to a series of messages hidden in Halo music tracks. There are thirteen reversed messages in the Halo series: one in Halo 2, three in Halo 3, 8 in Halo: Reach, and one in Halo 2: Anniversary.
Destroyer's Invocation Message
In the Halo 2 soundtrack Destroyer's Invocation, (the first movement of the Mausoleum Suite on the Halo 2: Original Soundtrack), a mysterious voice chants. If this mumbling is reversed (listen), a voice is heard saying:
The voice can be heard in the beginning cutscene of the level The Arbiter, the level Gravemind when John-117 rescues the Marines, and during the level The Great Journey when Thel 'Vadam battles Tartarus.
Black Tower Message
In the Halo 3 soundtrack, during the first few minutes of the track Black Tower, there is yet another message that can be heard if one reverses the music. Here is a transcript:
This message appears to be a reference to T.S. Elliot's Poem, The Hollow Men. The first part of the transcript refers to lines 20-23.
The second part refers to lines 47-49.
It is also worth noting that lines from the last stanza in T.S. Elliot's The Hollow Men were used in the Cortana Letters, which was repeated at end of the Halo 3 Announcement trailer and in Halo 3 itself.
Dread Intrusion Message
In the Halo 3 soundtrack, a reversed message can be heard in the tracks Gravemind, and Dread Intrusion. The entire message is currently unknown, however, several lines are intelligible. Oddly, the voice during these messages sounds different from Gravemind. It echoes considerably, rendering several lines unknown, it is much deeper in pitch, and multiple other voices can be heard. This could be a reference to multiple Graveminds from the past that speak as one because of inherited knowledge. The first two lines both have 7 syllables each, a possible 7 reference.
The exact person who is speaking is unknown. Just like the Destroyer's Invocation Message, there are many possibilities about who it could be. The likely candidates are the Gravemind and the Prophet of Truth, since only Elites and humans are mentioned.
Cryptic Whisper Message
In the Halo 2: Anniversary soundtrack Cryptic Whisper a mysterious voice chants. If this mumbling is reversed, a voice is heard saying: