As a former PianoDisc installer, it is my recollection that the PDS128
Plus has the Proteus sound generator installed in the control box that
plays floppy disks.

PianoDisc, if I remember correctly, has two forms of programs, one that
uses the Proteus sound generator and the other that has "Live" audio
recorded program.  One channel from the CD drives the piano and the
other channel is the live audio accompaniment.  The "Y" connector would
only send the piano signal.

If you are using the "Live" performance CDs, try sending one channel
from the CD player to the piano and the other channel to your stereo
system.  Might be worth a try.

Arthur Marino