sono oto no syoutai ga wakatta.

Here, の links two nouns.

• 音 = sound
• 正体 = true identity / what something really is

So 音の正体 means something like:

• the identity of the sound
• what that sound really was
• the source/real nature of that sound

This is a very common Japanese pattern: A の B = B of A or B related to A.

Examples:

• 車の色 = the color of the car
• 犯人の正体 = the criminal’s true identity
• 音の正体 = the true identity of the sound

正体（しょうたい） means true form, real identity, or what something really is.

In this sentence, it does not mean identity in the personal sense only. It can also mean the real nature of something mysterious or unclear.

So 音の正体 suggests that the sound was mysterious at first, and then the speaker realized what was actually causing it.

It has a slightly dramatic or revealing feel, like:

• what that sound really was
• the truth behind that sound
• the source of that sound

が marks 正体 as the thing that was understood/found out.

So:

• 音の正体が分かった = the identity of the sound became understood = I found out what that sound was

With 分かる, the thing understood is often marked by が.

Examples:

• 意味が分かる = understand the meaning
• 理由が分かる = understand the reason
• 答えが分かる = know the answer

In this sentence, 正体 is the piece of information that became clear.

分かった is the past form of 分かる.

Japanese often uses the past form when talking about a moment of realization:

• 分かる = to understand / to be understandable
• 分かった = understood / found out / realized

So 分かった here means that the realization has already happened.

This is very natural in Japanese. English learners sometimes expect a present tense like I understand, but Japanese often uses past tense for completed understanding:

• 今、分かった。 = I get it now. / I just realized.
• 原因が分かった。 = I found out the cause.

Because 分かる normally does not take を for the thing understood.

Instead, the thing understood is usually marked with が.

So these are natural:

• 意味が分かる
• 答えが分かる
• 気持ちが分かる
• 正体が分かった

For many learners, this feels strange because English uses a direct object:

• understand the meaning
• know the answer

But in Japanese, 分かる behaves differently. It is closer to become clear or be understandable than to a direct-object verb like eat or see.

That’s normal in Japanese. The subject is often omitted when it is obvious from context.

So その音の正体が分かった。 could mean:

• I found out what that sound was.
• We found out what that sound was.
• He/She found out what that sound was.

Japanese often leaves out:

• the subject
• the topic
• sometimes even the object

if the listener can figure them out from the situation.

So you should not expect every Japanese sentence to explicitly say I, we, or they.

Because the sentence is talking about that sound, and then about the identity of that sound.

So:

• その音の正体 = the identity of that sound

If you changed it to その正体, it would mean that identity or its true identity, which points to something different.

Compare:

• その音の正体 = what that sound really was
• その正体 = that true identity / its true form

In other words, その modifies 音, not 正体, because the sound is the thing being identified.

Yes. It sounds natural, and it is especially common in situations involving mystery, suspense, or discovery.

It is the kind of phrase you might see in:

• stories
• games
• anime/manga
• narration
• conversations about a strange noise

It has a slightly dramatic feel, because 正体 often appears when something hidden is finally revealed.

Similar expressions:

• 物音の正体 = the source/identity of a noise
• 犯人の正体 = the criminal’s true identity
• 謎の正体 = the truth behind the mystery

So this sentence sounds very natural if someone heard a strange sound and then figured out what caused it.

The structure is:

• その音の正体 = the identity of that sound
• が = marks what became clear
• 分かった = became understood / was found out

So literally, it is something like:

• The identity of that sound became known.

A natural English translation is often:

• I found out what that sound was.
• We figured out what that sound was.
• It became clear what that sound was.

This is a good example of how Japanese and English package information differently:

• Japanese: the identity of the sound became clear
• English: someone figured out what the sound was