Well, I'm glad they are open about this. A great time to update the passwords I use to more secure ones. :)
I would like to appeal to the community to look out for each other and report strange activities like spam posts or random content deletion to site admins, pointing out this recent hacking. Everybody who had a LibriVox account and used their password elsewhere should change it ASAP, especially if moderative or administrative accounts are involved!
... Let me just add a little bit of positive open content news though: I was looking for car blinker sounds, contacted three Freesound contributors and got permission from all of them to re-distribute under CC0, which I did on OpenGameArt. j1987 even added a "all my sounds are public domain" note. :)