One of my favorite low-tech ways to composite loops (Mac-only, but this could easily be replicated with different software in Windows or Linux):

  1. Play multiple sound files in QuickTime Player (set to loop with cmd-option)
  2. Control the mix with the volume sliders in QuickTime. You can even add a lo-fi backwards/forwards “scanning” effect by selecting a player and pressing command-left or command-right.
  3. Set Audio Hijack to record, with desired plugin effects added.
Screenshot of setup described in article

It has its limitations: It works best if your loops have silence at the start & end, otherwise there will be a brief pause between repeats.

For added fun: make multiple recordings like this, then feed those recordings back into the system again, ad infinitum.