So my computer was out of commission for about a year or so. I just got it running again (it was a RAM issue), and picked up an old video editing project that I'd been working on before the issue had cropped up. And now, for some reason, any playback audio in Premiere is just this high-pitched, trilling/chirping sound. The original audio from the clips is missing. (The audio does work fine in the original raw video files) When exporting, the issue persists in the exported file.
I'm using Windows 7, and the clips are video game scenes captured with FRAPS and Bandicam in .avi format. I've already updated Premiere. Tried it both with existing clips that were already in the project and with new clips, same thing. I tried normalising the gain, and it just made the noise louder. (Dunno if it's worth noting, but in the clips from Bandicam, the trilling noise isn't as loud as in the clips from FRAPS, but it's still present, and the original audio is still missing)
EDIT: For further reference, I've uploaded a comparison to YouTube.