The "Songs of Freedom and Captivity" were hymn-like pieces of narrative music sung by the Ood around the time of their great struggle on the Ood Sphere for liberation from human exploitation. Because of the complex nature of Ood song, however, it was not clear that they were actually separate songs, but rather one long story.
The Doctor could hear what might be called the "Song of Captivity" from the moment he arrived on the planet, but Donna Noble and other humans could not. Donna asked for the Doctor to allow her to hear it, but then could not bear its aching sadness.
After they had won their release, the Ood shifted the melody to something more uplifting. Although the Doctor would obviously have been able to detect this shift of tempo and tone, it was unclear whether Donna did. Nevertheless, when they departed the Ood Sphere, the Doctor and Donna — or as the ood put it, "the Doctor Donna" —knew the Ood were singing a "Song of Freedom". (TV: Planet of the Ood)
Behind the scenes
The song was never named within the narrative.
Composed by Murray Gold, and voiced by counter-tenor Mark Chambers, "Song of Freedom" is heard several times during Series 4. It is first heard during Planet of the Ood. The lyrics in the song are Latin.
In Journey's End an orchestral version with a full choir was heard during the sequence in which the Tenth Doctor and his companions use the TARDIS to restore the Earth to its original location. The vocals were sung by the Crouch End Festival Chorus. While the first version is heard by the characters, this version does not appear to be heard by them. It is not considered "in-universe".
Both versions of "Song of Freedom" are included in the Series 4 soundtrack CD issued in late 2008; the Planet of the Ood version as part of a medley entitled "Songs of Captivity and Freedom", and the Journey's End version on its own. In his liner notes Gold indicates that the arrangement used for Journey's End was influenced by the famous John Lennon recording "Give Peace a Chance."
A full-orchestral version was also performed at the close of the 2008 Doctor Who at the Proms concert.