David, as previous replies have indicated, when playing a 5.1 disc which has discrete center and surround channel content, modes such as DPLII, Neo:6 and Neural Surround have no function. They operate on 2-channel material to extract content from the front channels and direct it to the center or surrounds, as appropriate. With the multi-channel disc you were viewing, using Standard mode will play the DD or DTS content, whichever is present.
As to why the vocals were muddy, the location of the center speaker is important. It should be placed so that the tweeters are close to ear level, or are tilted so as to point toward the ears. It shouldn't be recessed, but rather should extend a fraction of an inch forward, so as to minimize reflection and diffraction effects. Then, there's the fact that dialog on many movie discs isn't mixed especially well.
