Hi Chris, thanks for the advice. It was the headphones after all. More specifically, it was the 1/4 in. adapter on my Audio Technica M50.
Luckily, my office has several pairs, so I tried the 1/4 inch adapter from a different pair and it worked! Look at the difference in the 2 connectors. This is from the same headphone model, but I bought them a couple years apart:
