Using the sound card of a DVD player with a surround sound system for my monitor [closed]

It's not a 'sound card' it's an analogue audio amplifier. Those cheap all-in-one 5.1-capable DVD players were never meant to play audio from any other source, so if you feed it at that point you're feeding the power amp, with no volume or EQ controls. You'd have to work that into your circuit earlier.

If you can figure out the pin-out of the ribbon cables [either by finding a service manual or by trial & error… which could be loud], you could feed it low-level 5.1, assuming you can get that from the computer. If you can't feed 5.1 directly from the computer without buying additional hardware, it would probably be cheaper & certainly easier to abandon this & just buy a cheap dedicated computer 5.1 system.