First, I used mplayer to rip the VOB files from the DVD. These files have trivial video in them, so that has to be removed. The command to do that is:
ffmpeg -i <infile.vob> -acodec pcm_s24le <outfile.wav>
The audio codec here is the usual one for 24-bit audio, as you can see if look at a wav generated from a flac file. E.g.:
# ffmpeg -i 01-statesboro_blues.wav
Input #0, wav, from '01-statesboro_blues.wav':
Duration: 00:04:19.94, bitrate: 4608 kb/s
Stream #0.0: Audio: pcm_s24le, 96000 Hz, 2 channels, s32, 4608 kb/s
At least one output file must be specified
Note that this is independent of the sampling rate, so it works for 48/24 files, as well.
So now I had a bunch of 96/24 wavs, which I could convert to flac in the usual way.