Dear Dorum,
even if in Part-View I would like to hear the whole Arrangement, not only the music of the particular part shown.
But:
How to…? 
THX
Best
C
That’s not possible, I’m afraid: Dorico only plays back the instruments that are in the active layout.
Depending on what you’re trying to do, you may prefer to work in the Full Score and then use Instrument Filters to focus on a specific instrument, rather than switching to the Part.
Another possibility, if your screen is big enough: Split the window, put the score in one half and the part in the other half. Play from the score while following the part.
Can you start playback in full score mode, then switch the window to the part while playback continues?
No, after a moment or two, as Dorico sends the playback data over to the audio engine, you’ll find that it starts playing only the sounds for the instruments in the layout.