Other forms: earwitnesses
An earwitness is someone who can give official testimony about something they've heard. If you're on a phone call and hear an argument break out in the background, you could become an earwitness to the disagreement.
The word earwitness usually refers to a person who heard something important and is required to talk about what they heard in court. But an earwitness may also just be someone who overheard something, even if it's not legally important. The word earwitness sounds like it might be a play on the more familiar word eyewitness, someone who saw something happen with their own eyes. But in fact, both words were first recorded back in the 1500s.