Fix/workaround for newer FFmpeg 1.1.3 and MP3 decoder

FFmpeg is now using S16P sampler for MP3 which is not actually
supported by audaspace, so request for S16 sampler instead.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54947 954f8c5b-7b00-dc11-b283-0030488c597c