Or rather: Background music and spoken sound is treated differently. Playing the MKV file in MPC-HC gives only the background music and sound effects, while playing it in VLC gives me both the spoken sound and the background music.
This is on the main title, which has a (single) 5.0 Dolby Digital soundtrack. For extra material, which has a 2-channel stereo soundtrack, everything is fine. Since makemkv is capable of treating the background sound and the narrative differently, it seems as if either the center channel (if that is where the narrative is encoded) or possibly the L/R channels (if the backgound and music is in the rear channels, the narrative in L/R) are treated differently.
The FAQ suggests that missing sound maybe due to the player not having the apporpriate codecs for the sound format. This is very unlikely to be the problem here; I can't imagine that the DVD uses different codecs for each channel! (I would be surprised if that is technically possible, too.)
There only a single sound option in the DVD disc info before I do the conversion, and only single option when playing back the MKV file (in either player).
I also tried to convert the MKV file to an MPEG4 file, using Replay Media Catcher. Again, the narrative was not included in the converted file. With two different MKV players both loosing the narrative, I strongly suspect that something has been messed up in the conversion.
I can play the file using VLC, but there are good reasons to use other players such as MPC-HC - e.g. VLC is "user friendly" to users capable of interpreting its source code to see how it operates. (For MPC-HC, a lower degree in software engineering is sufficient.)
For some reason, the Backup option is not available. I would appreciate knowing why.