I know this because the library default for channel 1 is Flute, and the track always starts out playing flute until i change it.
By default, Mixcraft has every track set to MIDI channel 1. Then you choose which tracks will play which instruments by assigning each track to the matching channels. I am using Edirol Orchestral libraries, and the way it is set up is that you can select 16 different instruments that will all be played from different channels. In Mixcraft I cannot find out how to do that, though i know the program has those capabilities due to an importing glitch I had once.
In my experience, most programs that use MIDI allow the user to assign each track to a certain MIDI channel (usually 1-16). I am having issues utilizing features of some of my libraries due to a possible short coming on Mixcraft.