This is something I've never changed, but I looked at it now. Try another MP3 file.
It also looks like you have to use the 'picker' and not just type the name.
edit: I just found out the madplay and mpg123 don't play WAV files. I ran into that once before and had to convert WAV to MP3 to use madplay. If the Audio plugin doesn't like your choice, it defaults to its own.
edit2: I tried different MP3 and it WORKS! If you mark the checkbox correctly, and use the 'picker' to select a new MP3 file, (do not type in the fullpath) THEN run the 'green triangle' test of Audio plugin, you will see just the Audio plugin run. Look in ... \Target\Win10PESE\Windows\Media\ which should then have your new MP3. If it is somehow not getting the new, replacement MP3 file, it always plays the original startup.mp3...
I use x64 source disk and also have the SysWow64 plugin Enabled.
[I tried uploading foghorn.mp3 but it is not an allowed file type]