MPEG-2 is an alternative compression format for audio, which may include multiple channels like Dolby Digital. However, MPEG-2 is not included in the specifications for NTSC DVD players, though some players in fact do support the format. But the format is considered "out-of-spec" for North America, so we have to reject any DVD master which uses this as the primary audio format.
Tech Tip: Prior to authoring your DVD project, check the default parameters for your encoding or authoring software to disable MPEG-2 audio, which seems to be the default for a small handful of software packages on the market today.