This is especially pronounced when one of the individuals has a strong foreign or regional accent. The individual with the less obvious accent will naturally try to mimic the stronger one.

A study with volunteers found that when the person with the weaker accent in the conversation tried to mimic the other, he or she was better able to understand the heavier accent (Richard Alleyne, "Mimicking a Foreign Accent Helps You Communicate," Telegraph.co.uk, Dec. 7, 2010).

So does this mean we can go around sounding like bad actors with funny accents? Probably not, but keeping an ear open for changing tones and pronunciations will help you understand others better. And we could all do with better communication!