I have been playing decent audio from my HP Pc onboard audio (Beats) via the line out to the aux in on my Sony STR-DE898 receiver. I thought I would crank it up a notch and install a Sound Blaster Z and use an optical cable. My problem is that there is no line out on the sound blaster. I tried using the line out L/R to the aux in but the sound seems muddled. I tried the optical cable to the MD Tape in but it only plays through the two front speakers.
MiniDisc was stereo so maybe the receiver is not programmed for digital surround sound on that input. Try and change the surround mode and if that doesn't work try a different optical input on the receiver. Also make sure that the PC is set to output 5.1 via optical.