If your dog trembled and shook at the sound of fireworks, he might have thought he heard gunshots. Heard means "perceived by the sense of hearing."
If you detected a sound using your ears, you heard it. Heard is the past tense of the verb hear, and it's also an adjective when you use it to describe a particular sound, as when you say, "The music was even lovelier since it was mysteriously heard from afar." A word you might confuse with heard is herd, which means "group of animals."