I used slackbuild.
I could install ffmpeg's dependencies (lame, x264) using sbopkg. but I couldn't install ffmpeg using "sbopkg -i ffmpeg"
it tried again and again many time to download and at the end, sbopkg failed to download and install ffmpeg.
anyway, I downloaded one from this page http://ffmpeg.gusari.org/static/
and extracted simply.
I found ffmpeg and ffprobe. so I copied them to /usr/bin. the previous ones already existed in there were deleted.
now, ffmpeg works, everything ok, my work is a bit dirty though.
anyway, thanks for ur post, I checked the pages u mention and downloaded opera-12.15.
I am going to learn what it is and keep it just in case.