What do you find upon deciphering this hidden message? A brief history of hidden Portal-related messages. The three image files encoded in the sound turn out to be slides detailing the ARG that led to the reveal of some Portal 2 info -- also using SSTV-coded images.
It turns out that said ARG had a budget of just $100, and used a BBS that was running on a vintage computer with a genuine 2400 BPS modem. You can see all three images in full here, and see (and hear) the coded message in Portal 2 after the break.