Sumthing Else Music Works and Microsoft have decided stuff all of the Halo music onto a massive compilation soundtrack.
There are five discs in total, and cover all three Halo shooters and compositions by Martin O'Donnell and Michael Salvatori. There's also a bonus DVD previewing five Halo Wars tracks, plus a behind the scenes video of the main theme being recorded.
Halo Trilogy - The Complete Original Soundtracks releases on Sumthing Else Music Works' digital outlet today. Only we couldn't find it there. Perhaps try later this afternoon when America wakes up.
Otherwise you'll have to wait until Monday 8th December, when Amazon UK will sell the bundle for GBP 39.49, which is quite a lot really. But then there's quite a lot of music on offer.