At least libogg, libvorbis, and libtheora build and run on iOS without modification in their current incarnations.
Experimental work in progress script to build the libraries to .framework bundles that can be easily linked to in an Xcode project: https://github.com/brion/OgvKit
- Must pass --disable-asm, as the asm code doesn't build for iOS (yet?)
iOS's native media framework is AVFoundation, which unfortunately is not extensible with codec or file type plugins. :(
It should be possible for an Obj-C wrapper library to use the same audio and video buffer types for input/output as AVFoundation does, however, which will make it easier to hook them up together for transcoding or live encoding from the camera.