Then your audio processor needs a thing called SIMULATED SURROUND.
In an honest-to-God AV Receiver, you switch to Dolby Plus, Dolby Prologic, Stereo Front+Rear, Stadium Mode, Concert Hall, others with similar names will take stereo audio signal and play on all your 5.1.
In an PC well... I notice my Mobo built-in Realtek ALC1150, the settings are anemic, I don't think it has any surround simulated modes.
Reason I said "Honest-to-God AV Receiver."