You will need to install a user script manager extension to install this script.
The LibriVox audiobook pages have links to the audio files at the start of each track. Clicking the link takes you to the audio file to play in the browser (or download, depending on browser settings). This script will replace the "Play" button of each track with an audio element loader. Clicking "load" will load that particular audio track in to an HTML audio element so that you can actually play the audio without leaving the page.