Why with some dvd players appears instantly the Dolby Digital sound in the home theatre but not with

I mean inmediately you insert the dvd you can check in the home theatre that Dolby Digital is sounded, but this does'nt happen with other DVD.

Probably the one that has Dolby Digital appears hasn't got DTS (the home theatre has it), what do you think do you agree?

Why with some dvd players appears instantly the Dolby Digital sound in the home theatre but not with others?shows

I believe it is because some DVD's dont have Digital sound only most newer 1's

Why with some dvd players appears instantly the Dolby Digital sound in the home theatre but not with others?comedy show opera theater

Most DVDs will default to Dolby Digital. If you want anything else, you have to select it from the setup menu on the DVD. This is entirely dependent on the way the DVD was programmed, so some may not default to DD, but I have yet to find one that doesn't (unless there is no DD soundtrack at all).
It isn't the DVD player but rather the DVD itself is at fault.

On some DVDs you have to tell it your preference for surround sound.
It's all about compatibility and different formats.