ad_ffmpeg: Set up audio format already during init.

It seems otherwise mencoder can get confused.

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@37206 b3059339-0415-0410-9bf9-f77b7e298cf2
mencoder: warn if audio format changes.

This is not handled in any way currently and will fail
completely.

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@37207 b3059339-0415-0410-9bf9-f77b7e298cf2