eumhyangi jeongmal johasseo.

음향 usually refers to sound quality, acoustics, or audio/sound system performance. In this sentence, it most likely means something like the sound or the acoustics were really good.

It is not exactly the same as 소리:

• 소리 = sound/noise/voice in a broad, everyday sense
• 음향 = audio quality, acoustics, technical or performance-related sound

So if you were talking about a concert, theater, cinema, or venue, 음향이 정말 좋았어 sounds very natural.

이 is the subject particle.

• 음향 ends in a consonant, so it takes 이
• if a noun ends in a vowel, it takes 가

So:

• 음향이 = the sound/acoustics + subject marker

In this sentence, 음향이 is the thing being described as good.

Yes. 음향은 정말 좋았어 is also possible, but the nuance changes.

• 음향이 정말 좋았어: focuses on the fact that the sound was good
• 음향은 정말 좋았어: makes 음향 the topic, often with a sense like as for the sound, it was really good

Using 은/는 can sometimes suggest contrast, depending on context:

• 음향은 정말 좋았어.
  = The sound was really good... maybe other things were not.

Using 이/가 is often more neutral when simply stating a fact.

정말 is an adverb meaning really.

It modifies 좋았어, so:

• 정말 좋았어 = was really good

It adds emphasis, like really, truly, or seriously in English.

You could remove it:

• 음향이 좋았어 = The sound was good.

Adding 정말 makes it stronger and more natural in conversation.

좋았어 comes from 좋다, which means to be good.

Breakdown:

• dictionary form: 좋다
• stem: 좋-
• past tense: 좋았-
• casual ending: -어
• result: 좋았어

So 좋았어 means was good.

A useful point for English speakers: Korean adjectives like 좋다 behave a lot like verbs in how they conjugate. So instead of using a separate word like was, Korean changes the adjective itself.

좋았어 is past tense, so it means was good.

This is natural when talking about a completed experience, such as:

• a concert
• a movie
• a performance
• a place you visited

For example, after a concert, you might say:

• 음향이 정말 좋았어.
  = The sound was really good.

Other forms:

• 음향이 정말 좋아. = The sound is really good. / I like the sound.
• 음향이 정말 좋아요. = polite version
• 음향이 정말 좋았어요. = polite past version

So the tense depends on whether you are describing something now or talking about a past experience.

좋았어 is casual, informal speech.

It is commonly used:

• with friends
• with people younger than you
• with people you are close to

More polite versions are:

• 음향이 정말 좋았어요. = polite
• 음향이 정말 좋았습니다. = formal

So if you are speaking to a teacher, stranger, customer, or someone you should be polite to, 좋았어요 would usually be safer.

Yes, it is completely natural and complete in Korean.

Korean often does not use dummy subjects like it the way English does. Since 음향이 already tells us what is being talked about, nothing else is needed.

So Korean says:

• 음향이 정말 좋았어.

English may translate it as:

• The sound was really good.
• The acoustics were really good.
• It had really good sound.

But Korean does not need an extra word like it.

Yes, you could, but the nuance is slightly different.

• 소리가 정말 좋았어 = The sound was really good.
• 음향이 정말 좋았어 = The acoustics/audio quality/sound system was really good.

소리 is broader and more everyday. 음향 sounds a bit more specific to audio quality or acoustics.

So at a concert venue, theater, or cinema, 음향이 정말 좋았어 often sounds more precise.

It is commonly pronounced close to 조아써.

What happens is:

• the ㅎ in 좋- weakens and disappears before a vowel
• so 좋았어 sounds like 조아써

This is why learners often hear it differently from how it is spelled.

Similarly:

• 좋아 sounds like 조아

This is very normal Korean pronunciation.