Official comment
You say "have the channels up but not hear the next track I am queuing up in a mix" -- as long as your crossfader is assigned properly, you shouldn't hear the track which is not assigned to the open side of the crossfader. But either way, my recommendation is to close the channel fader for any source not playing live.
As well, as @Infinite posts below, it could also be a matter of not properly configuring your audio devices within the rekordbox preferences.

