So, very good progress: some rotten vias for the card slots touched up by the battery holder, and a new holder installed after verifying traces. Very satisfying to be able to do that.
Now, onto the sound issue. I have a feeling that the sound may have a background hiss because it is loud. When I start up the machine, the startup sound is there, but way, way louder than I remember it. This is with the machine back inside its case, using the original speaker. I am not sure what about the cap replacement could have caused that to occur. I also am not sure how to adjust the volume on this thing, either!
There still is some intermittent crackling as well. I think there probably is some battery goop still somewhere causing havoc with the sound system. Now, where to look...
Already replaced the sound caps and have traced back to the headphone jack and inductor right there and cleaned them up.