If your Photo Cart is suddenly getting a 500 error or only displaying a blank page then you or your hosting company has updated the PHP (programming language) to PHP 7 which Photo Cart is not compatible with (and will not be compatible with) mainly because of the removal of the my_sql functions which are replaced with my_sqli functions.
All of a sudden, the audio to my slideshow is no longer working. I tried to upload a new file, but that doesn't work either. I notice that the size of the file only indicates KB. Before, I believe that it actually showed the size of the file.
I am using Photocart 6.1.3.
It is probably too large and not uploading properly. So after you try to upload it, you will need to FTP to your site and navigate to the Photo Cart -> slideshow_audio folder then also upload it there.