Saying AVI files run just fine on your system is meaningless.
What kind of AVI files? What codecs?
Are these DivX videos with MP3 audio?
Are they XviD videos with Ogg audio?
People can make AVI files with any moon-format codec they can think of,
and that would prevent it from working on most systems (unless they had
something installed to decode the format).
The extension ".AVI" doesn't mean anything.
Most of the AVI files I've compressed used XviD for video and Ogg for
audio. If you tried to play them, you may get video (if you had a
DivX/XviD compatible decoder installed), but you may not get any audio
unless you had something like an Ogg DirectShow filter installed.