If you want to copy an audio track there are two options, see below for details.
Select and Drop
Click an audio file on the timeline and select Duplicate.
With your mouse's cursor, hover underneath the scene where you want to add the audio file, and drop it by clicking.
You will now have the copied audio file in your video.
Select and Drag
This option is great if you want an audio file to repeat itself. First, place an audio file on the timeline, then immediately select the file again. The second copy will appear underneath the original waveform.
Then, click & drag the second file to the right of the first. You can drag it as close as possible without causing the waveforms to overlap.
The audio file will now play twice. You can repeat this process as many times as you like.