It depends on the format of the soundtrack and also on your speaker configuration.
Soundtracks fall broadly in to one of three categories:
Discrete surround - this would be DD, DTS, Dolby TrueHD, DTS Master Audio - so your amp should recognise each version from the bitstream data signal. 'Disrete' means there's an individual data string for each of the channels coded on the disc, so that could be anything from 7.2.4 for full fat ATMOS down to basic 1.0 Mono for an old movie transfer to Blu-ray or DVD.
Dolby Surround - a stereo signal with additional center and surround channel info encoded in a way that's undetectable to ordinary stereo gear but plays in surround when using the Dolby Pro Logic decoder
Stereo - does what it says on the tin
With the discrete formats you'd generally leave the amp to detect and play in its original form (Direct mode). The exception would be if you have a 7.1 speaker layout or front height speakers and you wanted to expand a 5.1 sound track to use those as well. That means playing DD or DTS with added Pro Logic processing (DPL IIx or DPL IIz) and most likely in Movie mode.
Audio in Dolby Surround comes mainly from stereo TV channels and some early DVDs, and maybe console games too that aren't in discrete format (I am not a gamer so someone else might be able to shed more light on this). For these you have vanilla Dolby Pro Logic or the later DLPII versions. They work in slightly different ways. DPL was only designed to be used with a Dolby Surround encoded signal. If you want to make a plain stereo signal in to pseudo-surround then you need to use DPLII. The Movie / Music / Game variants are similar to different RQ modes on an iPod or phone. Movie mode makes the most of the front/rear separation and effects steering. Music mode spreads the sound more evenly. Game mode is a more agrwssive version of movie with heightened treble and more bass.
For TV viewing DPLIImovie tends to work well.
Stereo signals can be processed with DPLII or DTS Neo or 7 Channel Stereo, or you could choose the ambient effects such as Jazz Club, Cathedral, Stadium etc.
