sae ieoponeul sseo bonikka eumak soriga jeongmal johayo.

Here, 새 means new.

Korean has two completely different words that happen to look the same:

• 새 = new
• 새 = bird

Context tells you which one it is. In 새 이어폰, it clearly means new earphones.

Both are possible, but they are not used in exactly the same way.

• 새 이어폰 = new earphones
• 새로운 이어폰 = new earphones, but often a bit more descriptive, formal, or emphatic

In everyday Korean, 새 + noun is very common for physical things that are newly bought or newly replaced:

• 새 차 = a new car
• 새 신발 = new shoes
• 새 이어폰 = new earphones

So 새 이어폰 sounds very natural here.

Yes—쓰다 has several meanings, and this is a very common source of confusion.

Depending on the object, 쓰다 can mean:

• to write
• to use
• to wear / put on
• to spend

In this sentence, 이어폰을 쓰다 means to use/wear earphones.

A few similar examples:

• 안경을 쓰다 = to wear glasses
• 모자를 쓰다 = to wear a hat
• 돈을 쓰다 = to spend money
• 편지를 쓰다 = to write a letter

So here it is not write. It is use/wear.

써 보니까 is made of:

• 쓰다 = to use/wear
• -아/어 보다 = to try doing something
• -니까 = when / since / after doing, I find that...

So:

• 써 보다 = to try using
• 써 보니까 = after trying to use it / when I tried using it, I found that...

This pattern often expresses first-hand discovery. The speaker is saying they learned something by experience.

So the nuance is not just I used the new earphones. It is more like:

After trying the new earphones, I found that the music sounds really good.

This is because of how Korean conjugates verbs ending in ㅡ.

For 쓰다:

• dictionary form: 쓰다
• stem: 쓰-
• add -어
• ㅡ drops, so 쓰어 becomes 써

So:

• 쓰다 → 써요
• 쓰다 → 써 보다
• 쓰다 → 써 보니까

This is a normal conjugation pattern for verbs/adjectives with ㅡ.

Because 보다 here is functioning as an auxiliary verb meaning to try.

So:

• 써 = use
• 보다 = try

Korean commonly writes this as two parts:

• 써 보다
• 먹어 보다
• 입어 보다

Then with -니까, it becomes:

• 써 보니까

So the spacing helps show that this is the pattern -아/어 보다.

Because 이어폰 is the direct object of 쓰다.

The speaker is using/trying the earphones, so Korean marks earphones with the object particle 을:

• 이어폰을 쓰다 = to use/wear earphones

That is why the sentence has 이어폰을.

Because the sentence is specifically talking about the sound of the music, not just the music in general.

• 음악이 좋아요 = the music is good
• 음악 소리가 좋아요 = the sound of the music is good

Here:

• 음악 소리 literally means music sound or the sound of the music
• -가 is attached to 소리, because 소리 is the thing being described as good

So the structure is:

• 음악 소리 = the sound of the music
• 음악 소리가 좋아요 = the sound of the music is good

This makes sense because earphones affect sound quality.

Using -가 puts focus on what is good: the sound.

So 음악 소리가 정말 좋아요 feels like: The sound of the music is really good.

If you used 소리는, it could sound more contrastive, like:

• 음악 소리는 좋은데... = The music sound is good, but... or
• 음악 소리는 좋아요 = As for the music sound, it’s good

In this sentence, the speaker is simply making a fresh observation, so -가 is very natural.

Because the speaker is describing the current result/state they notice after trying the earphones.

The trying happened first, but the judgment is about how the sound is now:

• 써 보니까 음악 소리가 정말 좋아요
  = After trying them, the music sounds really good.

If you said 좋았어요, it would sound more like:

• it was good at that time
• a past impression being reported afterward

Here, 좋아요 is more natural because the speaker is talking about a present quality they are experiencing.

Because Korean often leaves out subjects when they are obvious from context.

In this sentence, the implied subject is usually I:

• (제가/저는) 새 이어폰을 써 보니까...

But Korean does not need to say it if the listener can already understand who did the trying.

So the sentence naturally means something like: When I tried the new earphones, I found that the music sounds really good.

This kind of subject omission is extremely common in Korean.