The Halo 4 cutscenes, unlike the pre-rendered Halo Anniversary cutscenes, appear in-game in surround sound - and that's how we're making them available to you. (Thanks, as always, to Cody Miller for his stellar recording services.) If you don't have a surround sound rig, these degrade nicely to normal stereo.
As of March, 2016, these are now available in 1080p max resolution, and 60fps (with a couple of exceptions).
You can view a streaming version of the cutscene by clicking on a thumbnail.
Right-click (option-click on a mac) to download.