I would first try comparing the files to see if they are actually identical (you can do this in Windows 10+ by opening a command prompt and entering "CertUtil -hashfile <path to file> MD5
" for both files - if the same md5 code is returned the files are identical [technically, the hash could be identical and the files be different, but that's not something you need to worry about in this situation). If they are different, then the file is being corrupted *somewhere* along the line, My suspicion would be compression in the web server: try adding SetEnvIfNoCase Request_URI .(?:mp3|wav|wma|ogg)$ no-gzip dont-vary
to your .htaccess file before the ### WordPress begin block.
Are you being prompted to download the file or is the browser trying to play it for you?
Once you've downloaded the file, if you right click on it and select "Properties->Details", does the "(File) Type" show as "MP3 Audio File"?