I want to install ffmpeg, however it seems that its impossible without root. there are sources, but I have to compile it somehow. how to compile it? I just need ffmpeg "exec".
Easiest way is to use source package. Read this
However, as an alternative you can always try to download the binary package, uncompress the .deb package in your home directory and append
If you manually unpack binary packages, you need to resolve missing dependencies by your self (unpack also all required libraries).
Another option is to manually compile (