Extracting segment from .mpg file using ffmpeg; why does result have video but no audio?
Until very recently (2018-02-17), ffmpeg wasn't able to correctly remux pcm_dvd streams in MPEG-PS. You'll have to upgrade to a recent version. Get a git snapshot binary from FFmpeg (under More Downloading Options)

You can also just re-encode using -acodec pcm_s16be