This question was asked to me in my recent interview for a junior QA position.
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Your question is likely to be put on hold as it is opinion-based.
In general speakers can be tested from two general directions, functional and non-functional:
Functional test: you can test speakers based on its technical specifications, such as
This sounds like a generic 'how would you approach X' question.
So, what is important about speakers?
For anything you cannot think of first-hand, read the speaker manual. To add to that, Google known speaker problems.
Now you have a bit of a start that can be used to write test cases.
But, before putting a lot of time into detailed test cases, speak to the speaker product owner to verify what you have up until now makes sense and to ask about priorities and known problem areas.
In a real world case you would probably want to talk to the product owner first, to see what the current state of the speaker project is. In an interview you first start to tell what your seat-of-the pants approach would be.