A: Usually when people ask about the acidity/alkalinity of a product it's because they are interested in the product's archival properties. (Because of the use of acid-free papers in archiving documents.) So, I'm going to gear my answers toward archival applications.
Polypropylene sleeves are awesome for archiving. It's petroleum-based, so it takes a really long time to break down. (Oops, I mean bio-degrade.) It also doesn't off-gas -- which means it's not producing any gas that would hurt your disc or its label. And polypropylene is an excellent barrier against air and moisture. I can't find the pH for polypropylene but, since it's commonly used as the base material for pH testing strips, I'm going to guess it's neutral.
Tyvek sleeves, as you know, are moisture and tear resistant. It's silky smooth to protect against scratches. It's also opaque so it helps protect discs from sunlight. Museums and art dealers use Tyvek to protect paintings, sculptures, sketches, documents and other items during shipping and for storage. It's mold and mildew resistant and has a neutral pH of 7.0 (neither acidic or basic). Having said this, I'm hesitant to use this as an archival material. I've run across some chatter on message boards in the artistic community (particularly from "textile artists") about Tyvek crumbling -- sometimes in as little as four years. This is completely anecdotal, of course, but it makes me stop short of out-and-out recommending it.
Now, don't be too biased against our friend, the paper sleeve. I recently learned that most commercially produced paper is actually acid-free because paper mills are changing from china clay to chalk as the main filler material. Chalk is cheaper but it reacts with acids, so manufacturers have to make a chemically neutral or alkaline pulp to use the cheaper filler. The result is paper that doesn't degrade as quickly. Pretty cool, huh?
Also, US paper mills have to go away from acid-based paper because it pollutes lakes, rivers and streams. Most paper mills are located next to a water source and dispose their liquid waste directly into it. Too acidic and it kills fish and other good life in the waterway. Paper costs more to be made neutral but that's the trade off. Before you get too excited, most paper from China is still acid based.
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