Your original audio is not the same from beginning to end. That means you cannot join the front end to the back end without a noticeable "bump". There is a pitch change which makes it impossible to do that. So... I played around until I found an area of the track where the pitch was consistent. Now, that tells you what I did but not how I did it. So...How did you do it? (so I can do one as well)
Go to Control Properties > Play > Yellow Play. Check the box for Loop. There are two fields. In the upper field, choose Selection. In the lower field, choose 1000. Now, pushing the Yellow Play button will repeat a selected portion of an audio track 1000 times (unless you stop it, which you can do).
With your audio track selected, I started by clicking the Yellow Play button which caused the entire track to loop continuously while I moved the start cursor and the end cursor until I got the result I wanted. That identified the best start and stop points, so I stopped the playback and left the start and stop cursors where they were. I then used Ctrl C from the keyboard (clicking on Save will do the same thing). The exact and precise bit of audio needed is now saved (temporarily) in the Windows "clipboard".
Open a new Sound Window , accessible from the toolbar: New > Initial File Length (I just chose 1 second because the window will automatically stretch to fit).
The next Step is to just keep clicking Ctrl V ( or Paste from the toolbar) as many times as necessary to get a track of whatever length you desire. Now, that can get to be a pain in the... Finger... So just click Ctrl V and hold the keys down while watching the lower left corner of Goldwave -- You should see a small display that shows the time, which will continue to change as the length of your file grows with the constant repetition caused by holding the keys down. Release the keys when you get as much as you decide is appropriate.
Okay -- Actually doing it took a lot less time than composing this note to explain it. From start to finish, I had the loop completed in a couple of minutes.
Give it a shot --- And be ready to experiment and try things. That's the best way to learn what works and what doesn't, and it can be fun!