I’m still new at this so I hope more experienced sleeve conservators can chime in if I’m wrong:

First, do you mean an actual split seam, where the fold of the paper has weakened and torn, or just the adhesive drying out and the sleeve coming unglued?

The first one is probably the more complicated to fix, and I’m not sure of the best procedure. To avoid too visible repairs I might try to enforce the sleeve from the inside, but it’s going to be difficult depending on where the split is, relative to the opening.

Ungluing is easier, although I’ve only done this with albums where I don’t care much about the resale value: I’ve stuck a length of double sided scotch tape along the gluing surface to fixate the two sides. It sticks pretty well, and you don’t have to worry about drippy glue seeping out.

I can’t say if either of those approaches pass archival muster, but as far as keeping my own sleeves in somewhat working order, they’ve worked so far.

Edited to add: I dug around the internet and found a really thorough guide to restoring album covers. I sure got some inspiration to clean up some of my own thrift store finds!