@marklambie there are a few different variations even for the factory sound system You can read all about them on here [document|28951]. From looking through it, it appears as if all of them accept MP3 files. The MP3 file recording media supported by the radio are CDDA, CD-R, CD-RW, MP3, and CDDA+MP3. I suggest that you use CD-R and "burn" your files as MP3's to the CD. The rest will all depend on what computer OS you are using, what software you have etc. It would be far beyond the scope of this forum to get into those minute details.