Yes! You can loop it if you want. If you have gotten a Standard Sample License, you can loop your 15 seconds and produce your song with the sample looping through the whole or parts of the song. Just remember that your song has to be original, so be creative! You are not allowed to simply produce a loop from the original with none of your own input - your song must be substantially different from the original sample even if it is looped i.e. you have to write and produce completely new song.