Final answer:
The round instrument that produces sound when it is heard is likely a tambourine, a type of membranophone, which fits the criteria much better than the other options such as the saxophone, violin, or accordion.
Step-by-step explanation:
The round instrument that is heard in the context provided could be a tambourine or a drum, which are examples of membranophones. These are instruments that have a membrane which vibrates to produce sound. However, when specifically referring to an instrument characterized by its round shape and the fact it produces sound, the tambourine is the most fitting answer from the available options. The saxophone, violin, and accordion, while being musical instruments with distinct sounds, are not typically described as round instruments. The saxophone is a brasswind, the violin is a stringed instrument (chordophone), and the accordion is a free-reed aerophone with pleated bellows for air supply.