It's most definitely an issue with your footage sources, as you are using compressed media formats all over the place... Th error is a MediaCore one, but without more info on what actualyl your sources are and what you are specifically doing when the error is triggered, this is not going anywhere....
Yeah, and get rid of those mp3's!!! Replace them with aiff or wav files.
Since I started using AE with version 3, mp3's have always, Always, ALWAYS been toxic to AE. They've NEVER worked. And if you've gotten them to work without pops or clicks or other weirdness, you've just been lucky, and some day your luck will run out.