It seems that the FFmpeg program is not launching correctly, probably because of security restrictions that you have to configure (since you downloaded it from Internet).
Can you open the Ruby Console, copy/paste the following command and type Enter.
Kernel.system ""#{Traductor::FFmpeg.instantiate.program_locate}" -h"
This invoke FFmpeg help. The return value should be true.
If you get false, I would have a look at the previous post about giving permissions to FFmpeg.