I am Makarand P from the Dell Social Media and Communities team,
With regards to your issue, as rightly suggested by jefro, file formats have nothing to do with USB ports. If you want to convert the files from mp4 to mp3 format you could use a freeware called "Format Factory" and can be downloaded from the link below.
You could also try playing the files using VLC player since it can literally play almost any format ever designed.
Kindly revert for further clarifications.
Thanks and regards,
Dell Social Media and Communities Responder.