To verify: .minecraft/ffmpeg/bin/ffmpeg.exe
exists, right? If so:
Please post the contents of the fml-client-latest.log
file inside the log
folder inside your .minecraft
folder to some pastebin site and link to the post here.
Note that this log will only contain information on the last time you started Minecraft, not any previous runs.
Also, I'd prefer if you only used the edit post functionallity for fixing typos because I don't get any notification when you do.