We have great selection of opamps to choose from for mossing our personal soundcards these days. I can remember telling guys about LM4562's long before they were so "fashionable" They are great opamps but not everybody likes their sonic profile.
Their are other good audio opamps to try out:
..the list goes on.
When you compare the price of the LM4562 to the price of say a 5532 you see why companies choose the "tried and true" opamps. Profit and the designers personal preferences and design habits would come into play here. Either way, you can definitely improve on them. To note, some have been getting great sonic results by bypassing the opamp section entirely and bridging the outputs. I am not a advocate of this modification.
Caps are also area where you can modify but, you need to know exactly the right ones to change. You said, as AC doesn't require stable voltage, the components that drive the AC signal do..such as the DAC's and main supply for the card. There are also filtering caps in many audio designs. The particular caps that most change are the coupling caps sometimes referred to as DC blocks. These are usually on the inputs and outputs of any audio/amplification section. On soundcards they are changed on the outputs. These are in the signal path and can degrade the quality quite a bit. When the X-Meridians were released this was a common place to improve the signal quality. They were either bridged or replaced with better quality units as seen by a necessary evil by some. I also have a couple of friends who have bypassed the timing circuits on their soundcards using a precision timing board instead. I think the Xonar DX sounds quite good Stock, for the price. Any card can use some "tweaking"