A lot of multimedia player apps already include a repeat feature. It will be shown as 'loop' or 'repeat' -- as a couple of examples, with the MX Player Pro app as you're listening to an audio file that option with be shown as loop if you tap on the three-dot contextual menu in the upper right; with the VLC app when you're listening to a file the repeat feature has its own icon in the lower left, two arrows in a rotating shape. Tap on it one to repeat once, or tap again to look until you manually stop it.
Don't forget to minimize the app so just the audio is playing and the screen goes into sleep mode, and don't forget to keep that phone plugged into its power adapter.