hyogwa-eumi mudae dwieseo deullyeo.

Because 들려 comes from 들리다, not 듣다.

• 듣다 = to listen / to hear in the sense of actively hearing something
• 들리다 = to be heard / to come through audibly

In this sentence, the idea is not really someone is intentionally listening. It is more like:

• A sound effect is audible
• A sound effect can be heard
• I hear a sound effect

So 효과음이 무대 뒤에서 들려 sounds natural because the sound is simply reaching the listener.

Compare:

• 효과음을 들어 = (I) listen to the sound effect or hear the sound effect
  This treats 효과음 as the object.
• 효과음이 들려 = The sound effect is heard / I can hear the sound effect
  This treats 효과음 as the thing that becomes audible.

It is the casual present form of 들리다.

The steps are:

• dictionary form: 들리다
• stem: 들리-
• -어 attaches
• 들리어 contracts to 들려

So:

• 들리다 → 들려요 (polite)
• 들리다 → 들려 (casual/plain intimate)

This same kind of contraction happens in many verbs ending in -리다 / -기다 / -치다-type patterns when followed by -어/아요.

Because 효과음 is the grammatical subject of 들리다.

In this sentence:

• 효과음이 = the sound effect as the thing being heard
• 무대 뒤에서 = from behind the stage / behind the stage
• 들려 = is heard / can be heard

So the structure is basically:

• 효과음이 들려 = The sound effect is heard

If you used 을/를, you would be treating 효과음 as the object of an active verb like 듣다:

• 효과음을 들어 = (I) hear/listen to the sound effect

That is a different grammar pattern.

Because 에서 marks the place where something happens, or the source/location from which something is perceived.

Here, the sound is heard from behind the stage, so 무대 뒤에서 is very natural.

• 무대 뒤 = behind the stage / the back of the stage
• 무대 뒤에서 = from behind the stage or behind the stage as the place the sound is coming from

With sound, 에서 is common because it suggests the sound is occurring or coming from that location.

Compare:

• 무대 뒤에 사람이 있어. = There is a person behind the stage.
  Here 에 marks location/existence.
• 무대 뒤에서 소리가 들려. = A sound is coming from behind the stage.
  Here 에서 marks where the sound is coming from.

뒤 means back, behind, or the rear.

In 무대 뒤에서:

• 무대 = stage
• 뒤 = back/behind
• so 무대 뒤 = behind the stage or the back of the stage

It works very much like a location noun in Korean. You will see similar words often:

• 앞 = front
• 뒤 = back/behind
• 옆 = side/next to
• 안 = inside
• 밖 = outside

So 무대 뒤에서 is a very normal location phrase.

It is often taught that way, and that is helpful, but the full picture is a little broader.

You can think of it as:

• related to 듣다
• often translated like a passive: be heard
• but in real usage it often simply means to be audible or to come through to the ear

So in English, this sentence could be translated in several natural ways:

• A sound effect is heard from behind the stage.
• A sound effect can be heard from behind the stage.
• I hear a sound effect from behind the stage.
• There’s a sound effect coming from behind the stage.

All of those capture the idea.

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

English usually wants a subject like I:

• I hear a sound effect from behind the stage.

But Korean very often does not say 나는 unless it is needed for emphasis, contrast, or clarity.

So:

• 효과음이 무대 뒤에서 들려.

can naturally imply:

• I hear a sound effect from behind the stage
• You can hear a sound effect from behind the stage
• A sound effect is audible from behind the stage

The exact subject depends on context.

It is in a casual, non-polite style.

• 들려 = casual
• 들려요 = polite
• 들린다 = plain declarative, often used in narration, writing, or a neutral statement style

So the same idea could appear as:

• 효과음이 무대 뒤에서 들려. = casual
• 효과음이 무대 뒤에서 들려요. = polite
• 효과음이 무대 뒤에서 들린다. = plain statement/narration

A learner will often first meet -요 forms in conversation, so 들려요 may feel more familiar.

Both can mean roughly the same thing here, but the tone is different.

• 들려 feels conversational and immediate
• 들린다 feels more like a plain statement, narration, or observation

So:

• 효과음이 무대 뒤에서 들려.
  sounds like something someone says naturally in casual speech

• 효과음이 무대 뒤에서 들린다.
  could sound like narration, explanation, stage direction, or a more matter-of-fact observation

The basic meaning does not change much.

Yes, but the nuance changes.

• 효과음이 무대 뒤에서 들려.
  neutral observation: A sound effect is heard from behind the stage.

• 효과음은 무대 뒤에서 들려.
  puts 효과음 on topic, often with contrast or emphasis:
  As for the sound effect, it’s heard from behind the stage.

So 이/가 is the more neutral choice when you are simply noticing or stating what is being heard.

효과음 means sound effect.

It is made up of:

• 효과 = effect
• 음 = sound

So literally it is something like effect sound, which corresponds to sound effect in English.

You might see it in contexts like:

• movies
• theater
• radio drama
• games
• editing/audio production

In this sentence, it suggests a produced or intentional sound coming from backstage.

It can be either, depending on context.

The Korean present form often covers both:

• something happening now
• a general fact or habitual situation

So this sentence could mean:

• Right now, I hear a sound effect from behind the stage.
• Sound effects are heard from behind the stage.

Usually context tells you which one is meant. On its own, many learners would naturally read it as a present, immediate observation.

A very natural polite version is:

• 효과음이 무대 뒤에서 들려요.

That is probably what you would use in normal conversation with someone you are not speaking casually to.

You could also say:

• 무대 뒤에서 효과음이 들려요.

This just changes the word order slightly. Korean word order is flexible as long as the particles make the roles clear. Both are natural.