I record from the LP or 45 to the hard disk, now how do I use Nero Wave
Editor to PLAY,MIX,REMOVE HISS, AND RECORD, then save the new results to the hard disk? Nero Wave Editor doesn’t seem to allow you
to play and re-record at the same time. 
Play/record would cause re-digitization loss.
All other effects can be applied and then saved, but it does not appear to have any crossmixing capability. If you want to make a compilation, then chop the tracks up and put them together at the burn stage.
http://audacity.sourceforge.net - Not sure if this will do it, but it’s a much more general editor than Nero’s, and it’s free