Does Mixcraft have a "punch-in / punch-out" feature?
Mixcraft 6 and 7 do have a punch in / punch out feature. If you haven't checked out Mixcraft 7 yet, try it for free on the Mixcraft download page.
For older versions of Mixcraft, here's what you can do:
Remember in Mixcraft, you can always have an extra track to work with. Not all the sounds have to stay on the same track.