Audio used with this bot must be hosted online, such that the browser capture in OBS can find the file by URL. Files hosted on MyInstants.com can be used, but the URL is not the URL of the webpage with the button.
In order to retrieve the URL of the actual sound file, you will need to be on the page for the sound, such as the one pictured here
You will then need to right click on the Download MP3 button and find the option to copy the link. It may be called "Copy Link" or "Copy Link Address" or something similar, depending on your browser. Selecting this will copy the link for the sound file. From there, it is easy to paste the link into the bot. Go do your config page and simply paste the URL into the field for the appropriate chat action.
Finally, press the Play button for that sound to assure that the file is correctly linked.
Enjoy your new sounds, and happy streaming!