I hope this post helps someone like me in the future.
A while back, almost all my WAV files started playing back in Audacity with moments of distortion – a little like they’d been run through a vocoder for a fraction of a second. This was especially alarming because most of the files I play are generated by an audio utility I’m developing, so I assumed I’d broken something in my own code. But it happened with known-good files, and VLC played them all back correctly. Something was wrong with Audacity.
Long story short, I had forgotten I’d enabled the “Software playthrough of input” option in Audacity’s Recording preferences. I assume this causes Audacity to feed mic input into the output, even when not recording. No matter what the root cause was, disabling the option fixed the distortion, and I’m happy again.