When I tried ldd /usr/bin/ffmpeg,

root@ARSENALFANS:~/Desktop# ldd /usr/bin/ffmpeg
linux-gate.so.1 => (0xffffe000)
libavformat.so.51 => /usr/lib/libavformat.so.51 (0xb7eb1000)
libavcodec.so.51 => /usr/lib/libavcodec.so.51 (0xb79b1000)
libavutil.so.49 => /usr/lib/libavutil.so.49 (0xb79a6000)
libm.so.6 => /lib/libm.so.6 (0xb797f000)
libpthread.so.0 => /lib/libpthread.so.0 (0xb7967000)
libc.so.6 => /lib/libc.so.6 (0xb7825000)
libz.so.1 => /usr/lib/libz.so.1 (0xb7811000)
libogg.so.0 => /usr/lib/libogg.so.0 (0xb780c000)
libdl.so.2 => /lib/libdl.so.2 (0xb7808000)
liba52.so.0 => not found
libfaac.so.0 => not found
libfaad.so.0 => not found
libmp3lame.so.0 => not found
libtheora.so.0 => /usr/lib/libtheora.so.0 (0xb77ce000)
libvorbis.so.0 => /usr/lib/libvorbis.so.0 (0xb77a5000)
libvorbisenc.so.2 => /usr/lib/libvorbisenc.so.2 (0xb76a7000)
/lib/ld-linux.so.2 (0xb7f65000)
libx264.so.56 => not found
libxvidcore.so.4 => not found

The problem is,how I get to fulfill the libraries that I don't have?

iirc all the libraries exist as SlackBuilds so all you need to do is download what you need and install.

Why think that something is missing ??
'ldd' just tells you which libraries your application is using.
Quote from wikipedia: 'ldd List Dynamic Dependencies, Unix program'
Good luck with ffmpeg.

How did you get ffmpeg? Do the same thing for lame, a52dec, x264. faac, faad and xvid...

from slackbuilds:

You may have to build ffmpeg again after installing the dependencies (I'm not positive though -- it depends where ffmpeg looks for them by default). As said above, the easiest way is to just grab SlackBuilds for ffmpeg and its dependencies from slackbuilds.org.