General:
* Audio: Relating to sound, especially recorded sound
* Audience: A group of people gathered to hear or see something
* Auditorium: A large room or hall for public gatherings, especially for watching performances or listening to lectures
* Audition: A trial performance to assess someone's suitability for a role
* Auditory: Relating to the sense of hearing
* Audacious: Bold, daring, or adventurous
* Audiophile: A lover of high-quality sound reproduction
* Audiologist: A healthcare professional specializing in the diagnosis and treatment of hearing and balance disorders
* Audiometer: A device used to measure hearing ability
Less common, but still related:
* Aural: Relating to the ear or hearing
* Aura: A distinctive atmosphere or quality surrounding a person or place
* Auscultation: The act of listening to sounds within the body, typically with a stethoscope
* Audibility: The quality of being able to be heard
* Audible: Capable of being heard
This is not an exhaustive list, but it gives you a good starting point for understanding how the root "aud" is used in English words.