Certainly. Import your track midi/audio. Press button "Loop on/off" and then set you loop points in the windows or drag the markers to where you want the loop to start and finish
Or use the Loop shuttle command. The Help file is pretty good at describing how to do this
It will keep playing and repeating the audio until you tell it to stop. If its audio, you can do all sorts of neat tricks in Sonar. Like Groove enable it, which then makes it easier to lengthen the clip/clips to the given length of a song. And to more easily recognise the loop point in the imported clip, so you know where to put your end marker so that it smoothly ends and goes back to the begining again, seamlessly playing.