Most DVD-Video discs contain Dolby Digital soundtracks. However, it's not required. Some discs, especially those containing only audio, have PCM tracks. It's possible --but rare-- for a 625/50 (PAL) disc to contain only MPEG audio. Discs with DTS audio are required to also include a Dolby Digital audio track (in a few rare cases they have a PCM track). See
1.32 for more on DTS.