@ECHO OFF
FOR %f IN (*.mkv) DO call ffmpeg.exe -i "%f" -c:v copy -c:a libvo_aacenc -b:a 256k "%f"

[NULL @ 0000000005a3eaa0] Unable to find a suitable output format for 'pipe:'
pipe:: Invalid argument
ERROR: CoreAudioToolbox.dll: The specified module could not be found.