Not the best release.
I'll have to install Mafia from my old CD's so I can take a look at this.
If the music tracks have been converted to OGG (Vorbis) files in a Music folder then they've probably used extracted those songs from AB.DTA.
It should be easy enough to re-add the missing tracks using the available tools...hopefully http://www.kamalook.de/Mafia/DTA.html EDIT:
Using the mafia file extractor - I can see what GOG have done.
They've basically included blank city_music_xx.ogg files instead of the real ones from the disks.
I'll upload the original files.