Apparently they're from an event that went down in March to celebrate the 10th anniversary of Zim's premiere, called InvaderCON. Four of the original voice actors were there, and they were awesome enough to do a read-through of two unproduced scripts. From left to right they're: Richard Horvitz, the voice of Zim himself; Andy Berman, the voice of Dib; Melissa Fahn, the voice of Gaz; and Rikki Simmons, the voice of GIR. You'll notice that GIR doesn't sound quite the same without the processing the voice normally gets, but it's still undeniably him.
The first script, "Day of Da Spookies," deals with Zim trying to discredit Dib and the reemergence of Skoodge.
The second, "Mopiness of Doom," sees Dib giving up on his pursuit of Zim, and the consequences.
Like I said, I don't think the show will ever be resurrected, but perhaps there will be that tantalizing bit of hope for a while, as this bittersweet blogpost from creator Jhonen Vasquez illustrates. But I thought those readings were pretty great to see, and perhaps offer a bit of closure for us doofuses who have become so bitterly impassioned over a dumb cartoon?