There has changed a lot since this library was contributed.
I'd not use that version of it anymore as it brings its own outdated secondary libraries which weren't well ported back then.
So I'd rather add the two .h/.cpp files and copy/paste the contents of them into these files and then add the secondary libraries (if needed) via the library manager.
However, if you only want to playback a fix set of MP3s, there would be a cheaper/simpler module
There are also two libraries available in for that module