Hi,
Actually Notation Software products don't have their own sound library, but they can use whatever you have available For example, the GS Wavetable is the default just because it's on every Windows machine out there, but we have users who hook up their PCs to keyboard synthesizers, use VST Host (or something similar) to use VSTis, Garritan Libraries, EastWest Orchestra, etc.
The blessing (and some would say curse, at least while still on the learning curve
) is that these various devices have a lot of different control parameters to help with articulations and other nuances of "live" playback feel for a file. They may not sound as good with a "flat" (ie. no MIDI Controllers going) file, but if the MIDI cc's - accessed using Graph Over Notes in Notation Composer - are used, you can get often get a performance that sounds pretty close to live musicians.
ttfn,
Sherry