they were taking part in an unusual experiment, which involved tracking their own voices over time. This was done by making 10-minute recordings every few weeks. They would sit in front of a microphone and repeat the same 29 words as they appeared on a computer screen. Food. Coffee. Hid. Airflow.

One of those changes was the "ou" sound in words such as "flow" and "sew" that shifted towards the front of the vocal tract.

I'm not actually sure what sound change they're describing there. Can anyone explain with examples or IPA?

edit: Cheers for the answers (turns out I misunderstood which part is the vocal tract)