I'm trying to get sound from my PC sound card into my sterio amplifier.
I've got a connector cable with two male plugs on each end.
One end goes into my sterio amplier's auxilery-1 female
connectors and the other end must go into the PC sound card's
female connectors on my PC's motherboard.
The PC sound card connectors are in two rows of 3 female connectors.

white black off-red
red green blue

My two male connectors are black and red.
Does anyone know what connects to what so I can get
the sound card's sound into my sterio ?
I seem have lost the motherboard's manual.

Whatever mixer ubuntu uses which I figure pulseaudio.

Make sure and pick the card as the input device. It is probably defaulting to default sound card jack.

Speaking from experience.

It depends on the age of the motherboard / sound card and if it adheres to the standard design guide.

https://en.wikipedia.org/wiki/PC_System_Design_Guide

I would assume the off red is actually pink and the only color that does not match is white.

Is this your cable? Are they TRS i.e have two black rings or one? TRS = Tip, Ring, Shield which is a stereo cable or a single band for mono.

Is this a real add on sound card or built in sound?