I have problems with all of the .mp4 files I'am working with. Firstly, when using them in composition, can't get audio preview. But what is more frustrating is that when I close AE and reopen the project I am working on, it simply says : "After Effects warning: 4 files are missing since you last saved project". Of course all of those 4 files are .mp4 and I haven't moved or renamed any of them. All other media files(.mp3, .avi, .jpg) remain intact. I even tried right click->Replace Footage->File... and when I choose that same source file I get another error message saying:
I tried following:
-moving source .mp4 files to various locations, including C:\Videos\
-importing the same files, from the very same location (it worked but just until I reopened AE)
I don't have much experience in AE, been using it for few months so my apologies if it is usual error.
Windows 7 Ultimate(x64)
AMD Phenom IIx6;8GB DDR3
After Effects 188.8.131.528
Don't use MP4 files in After Effects. MP4 is a delivery codec, not something to be used in compositing. Convert the files to a decent intermediate codec, like Quicktime Animation, Quicktime PNG, or a lossless AVI.
Clear the Media Cache from the prefs. Also update your AE to the latest 11.0.2. version.
Heed Mylenium's advice. Install the updates. Choose Help > Updates.