English is spoken in Australia (by people).

Explanation: In the passive voice, "speak" changes to "is spoken," and we don't need to use the "by" phrase here since it's a general statement about what happens in Australia.
Rice is grown in many places (by people).

Explanation: "Grow" is in the present simple, so it becomes "is grown" in the passive voice. The "by" phrase is optional because it’s a general statement about where rice is grown.
This machine was made long ago (by somebody).

Explanation: "Made" becomes "was made" (past simple passive). The "by somebody" phrase is not necessary because the focus is on the machine being made, not the person who made it.
The phonograph was invented by Edison.

Explanation: "Invented" becomes "was invented" (past simple passive), and the "by" phrase is necessary to indicate the inventor (Edison).
The bell is rung every hour (by someone).

Explanation: "Rings" becomes "is rung" in the present simple passive. The "by" phrase is optional but can be included if needed for clarity.
General explanation:

In the passive voice, we focus on the action and the recipient of the action, not the doer. The structure is formed with the appropriate form of "to be" (am, is, are, was, were, etc.) + the past participle of the verb. The "by" phrase is used when it is important to mention who performed the action, but it’s often omitted when the doer is unknown or not important.