The sound driver is
written to play back music in stereo from SEPARATE files for EACH channel. That's why you have separate monaural files for the left and right channel.
Problem is, that works fine on the original GD-ROM, but a CDR just can't be read fast enough by the DC to play both files simultaneously (it's been tried), so for that reason the playback was left at monaural (I think they merged both channels into a single file each, and left it at that).
There was a web page with more details about this, unfortunately I don't remember where it was... Google might be your friend.
