If Rapture Pro is like Rapture, it can't loop a sample. You can create a loop in the wav file (your audiofile) using a wav editor or create the loop in an sfz file that refers to your wav file. Then the segment of the file that is looped will loop as long as a key is down. If I understand things correctly, the loop and sustain points in Rapture's envelopes only loop the action of an envelope, not the audio that the envelope is operating on. Rapture's loops are not intuitive and there is very little discussion of them in the forum archive. I haven't had much success with them the times I've tried. If anyone has a clear idea of what's going on, I'd like to hear it.
You could also loop the entire sample by turning it into an audio loop in Sonar track view (Ctl-L), with or without having used Rapture Pro in between to modify the sample.