[open source wanted]: display multimedia files (avi, jpeg, mpg) etc

Why would anyone want to implement these in assembly language? Rendering these formats is a highly complex matter and implementing that in assembly language would be an enormuous effort that would hardly yield any noticeable performance improvement at all.

There's a bunch of open source implementaions in C/C++ available, probably in other high-level languages as well. Why not use these?

If someone would have imlemented that in assembly language, that would be a highly specialized custom solution that certainly wouldn't be released as open source. And even if you could get it, you probably wouldn't really be happy with it, because it would be inflexible due to its high degree of specialization.