Above is from the manual
As you can see MP3 is not one of the formats it expects. You can convert MP3 to MP4 if you really want to go to the trouble.
This was just the first one I found.
You have to upload the MP3 file then once it finishes you download the mp4. I just tried one to see if it worked. Apparently you can do multiple files at one time and download them all at one time as a zip. I'm hard of hearing so can't speak to the quality of the conversion but I was able to play the resulting mp4 file in VLC. You just get a black screen. Probably if you look around you can find a way to add still pictures. This one looks promising tho I haven't tried it: