ffmpeg convert dvd vob to mp4 sometimes no audio

Using this cmd (from here : https://www.winxdvd.com/dvd-ripper/rip-dvd-with-ffmpeg.htm) to convert dvd vob files to mp4. (adjusting file names as needed)

ffmpeg -i "concat:VTS_01_1.VOB|VTS_01_2.VOB|VTS_01_3.VOB" -b:v 1500k -r 30 -vcodec h264 -strict -2 -acodec aac -ar 44100 -f mp4 convert.mp4

It has worked before on multiple dvds but now I’ve come to a dvd where the resulting mp4 has no audio. (I have confirmed that there is audio by playing the dvd in dvd player)

Is there something missing?

Cheers.