You just link your PC's soundcard outputs to HIFI's external input. If your sound card has optical output and HIFI has optical input, that would be super.
Cheaper option, Put MP3 files to a CDR or DVD and play them on DVD player. Most domestic stand alone DVD player can play MP3 files. If you sony cannot do it, you can get a cheapr one about £30.
If you have deep pocket, you can get a Apple IPOD or other Hard Disk based MP3 player and put all you MP3 files to it and link the payer's output (earphone socket) to your HIFI.
If you have even bigger pocket, build a small monitorless PC box as a standard MP3/Media files player and link to HIFI.