When you hear 'sound out', don't you picture percussion activities?

Question

While they were eating it, Michael risked asking Howl what the King had wanted.
“Nothing definite yet,” Howl said gloomily. “But he was sounding me out about his brother, quite ominously.”(Howl’s Moving Castle, by Diana Wynne Jones)

sound out: to try to find out from somebody what they think about something, often in an indirect way (OALD)

When I read the passage I thought, 타진하다 [tajin-hada], for ‘sound out.’ (I can find that Japanese use the same word, だしん [dashiŋ]; 打診). And OLAD’s Korean translation also used the word, 打診. When I hear the Korean equivalent, 타진하다, I picture the image or sound from an activity of percussion or tapping, e.g. by a physician, even though in the politic news in which the word is used as an idiom.
I hear too much ‘it’s an idiom, no argue at all! Just accept it as it is!” These are not a few sayings to hear even in this LEARNING website. QUESTION (1): Don’t your teachers in classrooms say about the word’s backgrounds, even though it’s from just their pure imagination, not based on grounds?
QUESTION (2): Where the words, sound out, came from? Don’t you recall any images or sounds as I recall, when you hear the sounds, at all?

Answer 1

The term "to sound out" means to test the depth of a body of water, for example to find out if it deep enough to maneuver a boat or ship in. By extension, we use it to mean testing how another person reacts to an idea.

According to Wikipedia,

"Sounding" derives from the Old English sund, meaning swimming, water, sea; it is not related to the word sound in the sense of noise or tones.

So it's really only coincidence that this term sounds like it has something to do with noises. However this coincidence may have contributed to the common figurative usage.

As for your first question,

Don’t your teachers in classrooms say about the word’s backgrounds, even though it’s from just their pure imagination, not based on grounds?

A good teacher wouldn't speculate about the origins of a word. However, it is often important to know word origins and the information is readily available. Our dictionaries often give the etymology (or origin) of many words. There are also specialized etymological dictionaries that focus on this information. One very accessible one is the Online Etymology Dictionary.

Answer 2

In the context of the sentence in OP's question, I think the following definition is better suited.

sound somebody/something ↔ out phrasal verb

to talk to someone in order to find out what they think about a plan or idea: He sounded people out and found the responses favourable.

They want to sound out his opinion before they approach him formally.

sound somebody/something ↔ out about

I wanted to sound her out about a job that I'm thinking of applying for.

from: Longman Dictionary of Contemporary English

Answer 3

I'm a native AmE speaker and to me, myself, personally, to "sound someone out" sort of means to hear something percussive, because I think of it as being like using radar or echolocation with ideas.

I say something, the other person hears & considers it, and his response is the reflection my statement has created from his thinking. The idiom "to bounce an idea off of (someone)" is very similar in meaning, and gives me the same sort of mental image.