In article 20050313161307.GB21955@wotan.suse.de you write:
DVD's in the PAL/SECAM area can include MP2 (MPEG-1 layer II) tracks, as well: http://www.discusa.com/technology/dvdvideo/dvdvid_audenc.htm
I've a few of DVD's of this area code. No one of them uses MP2, many of DTS and some Video DVD with Music PCM and DTS 24/96. AFAIK DVD have to had an AC3 track accordingly to the specs. The rest is optional.
According to http://www.dvddemystified.com/dvdfaq.html#3.6, the specs are as follows:
"Discs containing 525/60 (NTSC) video must use PCM or Dolby Digital on at least one track. Discs containing 625/50 (PAL/SECAM) video must use PCM or MPEG audio or Dolby Digital on at least one track. Additional tracks may be in any format."
So anything which is produced for more than one market will likely have AC3 as the smallest common denominator. (But recordings from DVB broadcast in PAL format which contain only MP2 audio make "valid" DVDs too. What do US broadcasters use?)
Olaf