This might seem trivial but check the line-out connector first, it's actually the biggest point of wear on one of these cards. Entirely possible for one of the contacts to get bent & no longer make contact or even a solder pad to break over 20 years of repeated plugging & unplugging.

Other than that it's hard to tell anything from such a dark/unfocused picture. The pins at the bottom of the Yamaha chip look rusty but that could just be lighting.

I would also start with checking the connector. Cut a 1/8" stereo cable (like an aux cable) in half, strip the wires, plug it in, use a multimeter to measure resistance/continuity between those stripped wires and the solder pads of the jack. Should be fine if it reads less than a few ohms but anything above that would be a bad connection from oxidation or weakened spring pressure in the connector. As close to 0 as possible is best but it won't be spot on 0 either.

Edit to note: Oh and if you do get a reading when doing that, move onto twisting the connector while it's plugged in. The resistance should not change much at all, and should not jump to a high value, even for a split second. If you get even a split second of a high resistance or non-reading ("OL" - overload, or infinity, which mean the same thing) then it's the jack.