ad_dvdpcm reads MPEG specific headers directly (passed through codecdata by demux_mpg), so you couldn't use ffmpeg's "pcm_dvd" with demux_mpg. Change demux_mpg to set the correct audio parameters directly. The code for this is taken from ad_dvdpcm. ad_dvdpcm is evil because it still does partial packet reads (with demux_read_data()), and it's redundant to libavcodec anyway.