if i have a song with four voices ( soprano, contralto etc) there's a way to hear only a voice (f.e. contralto?)
Yes.
(1) If the voices are on separate staves (staffs): Click on the specific staff, to select it. Use SHIFT F5 to play only that staff.
When I use layering, I find it helpful to un-layer before playing an individual staff.
(2) If the voices are combined as chords on a single staff, then you need to select (presumably) the second-highest note that is playing at any one time. This is a technical problem (it would be a technical problem in any program, not just NWC). You can go to the Scriptorium and see of the "chord splitter" helps. In NWC2, it might be possible to write a script that could distinguish the necessary notes.
You can also use the Mute List feature to selectively play certain parts.
For part extraction, Andrew Purdam's script for this is included in the latest NWC2 User Tool Starter Kit (http://www.noteworthysoftware.com/nwc2/usertools/). See news headline #32 (http://www.noteworthysoftware.com/news/32.htm) for details.
When preparing a training CD, I put each featured voice on the right channel (midi pan 127), keeping everything else on the left (pan 0). By changing just the balance controls the user can highlight his or her own part as needed.