Thank you all for your prompt responses.
Richard, I think you nailed it – I’m not sure at what point it happened, but it does look like the two channels were canceling each other out; separating them took care of it. Can’t believe I didn’t notice it.
Thanks so much for what turned out to be a simple fix.
For my education, do any of you know why this problem was extremely noticeable over the terrible computer speakers but wasn’t noticeable at all over headphones or better speakers? Do the laptop speakers mix together stereo tracks in a way that headphones don’t?