Yeah unfortunately there isn't a lot of control over HTML5 audio AFAIK. There is a preload option but it's specific to the individual <audio> element.

Maybe as a hack I could change it so each article has a hidden <audio> somewhere that preloads it in the background, then when you actually play back it should use cache.

I don't know if that's wise over a cell connection though, downloading all the MP3s.

Perhaps a better approach would be to add a "preload" button, and then use the hidden element as you suggested so we aren't loading audio we aren't interested in.

Another idea would be to add a personal user queue to do the same thing. Awesome site btw.