ffmpeg multithread

[gogogo]

Hello,
I have a 8 core debian vps under xenserver with 64gb of ram, and the ffmpeg process use only one core to transcode the video file, we use it improperly to view preview of our video works.
I have tried to add the option -threads 8 to the ffmpeg transcoding in the subsonic setting but nothing happen on the ffmpeg process..

for example, for the mp3: ffmpeg -threads 8 -i %s -map 0:0 -b:a %bk -v 0 -f mp3 -