the subtractor is not a loop player, it is a synth. It will only play its own sound files.
if you need to play audio loops you should try the other soft-units.
In the NN19 or NNXT there are 2 ways to play audio files : either load a PATCH with several files (a few octaves of piano, for example), or load a SAMPLE into the synth and trigger that sample every time you want to start the loop again.
It's a bit hard to get the loop to "loop" right, for tempo issues, but if you know what you want ahead of time, you can create the right kind of audio file and have it loop again and again.
The best way to go is the Dr.Rex, but you need the Recycle software for that.
You could also try the "ReDrum" machine. Just load a wave file (your loop) into the drum machine and play it again and again, while tweaking the pitch knob and changing the tempo to have the loop fit in the song's tempo.
That's about it, off the top of my head. Hit me up if you've got more problems
Peace.
