Korkunç bir ses duydum.

Turkish word order is often subject–object–verb, while English is usually subject–verb–object.

So this sentence is literally arranged like:

• korkunç = terrible
• bir = a / one
• ses = sound, noise, voice
• duydum = I heard

So the structure is basically:

terrible a sound heard-I

In natural English, we rearrange that as I heard a terrible sound.

Also, Turkish often leaves out the subject pronoun when the verb already shows who did the action, so there is no separate ben for I here.

Here, bir works like the English article a/an:

• bir ses = a sound

It can also literally mean one in other contexts, but in this sentence it is functioning like an indefinite article.

So:

• ses = sound
• bir ses = a sound
• korkunç bir ses = a terrible sound

In Turkish, adjectives normally come before the noun, just like in English.

So:

• korkunç ses = terrible sound
• güzel ev = beautiful house
• büyük araba = big car

That part is actually quite familiar for English speakers.

No. Turkish adjectives do not change for number, gender, or case the way they do in many European languages.

So korkunç stays korkunç no matter what noun it describes:

• korkunç ses = terrible sound
• korkunç sesler = terrible sounds
• korkunç bir olay = a terrible event

This is simpler than in languages where adjectives have to agree with the noun.

Duydum comes from the verb duymak, which means to hear.

It breaks down like this:

• duy- = hear
• -du = past tense
• -m = I

So:

• duydum = I heard

Other examples:

• duydun = you heard
• duydu = he/she/it heard
• duyduk = we heard

Because duymak is the dictionary form, meaning to hear.

When you actually use the verb in a sentence, you conjugate it.

Here the sentence is in the past tense, so duymak becomes duydum = I heard.

Compare:

• duymak = to hear
• duyuyorum = I am hearing / I hear
• duydum = I heard
• duyacağım = I will hear

So duydum is the correctly conjugated form for I heard.

The Turkish past tense suffix has different forms because of vowel harmony:

• -dı
• -di
• -du
• -dü

Which one appears depends on the vowel before it.

In duy-, the relevant vowel is u, so the past suffix becomes -du:

• duy + du + m = duydum

This is a normal vowel harmony pattern in Turkish.

Because the verb ending already tells you the subject.

In duydum, the final -m shows I.

So Turkish often omits the pronoun when it is obvious:

• duydum = I heard
• duydun = you heard
• duydu = he/she/it heard

You could say Ben korkunç bir ses duydum, but ben is usually unnecessary unless you want emphasis or contrast.

This is an important Turkish grammar point.

In Turkish, a specific/definite direct object usually takes the accusative ending, while a nonspecific/indefinite direct object usually does not.

So:

• Korkunç bir ses duydum. = I heard a terrible sound.
  
  • nonspecific, indefinite

But:

• Korkunç sesi duydum. = I heard the terrible sound / that terrible sound.
  
  • specific, definite

And also:

• Korkunç bir sesi duydum. can mean something like I heard a certain terrible sound or I heard one terrible sound in particular, depending on context.

So the lack of accusative marking here fits the meaning a terrible sound, not the terrible sound.

Ses is a flexible word. It can mean:

• sound
• noise
• voice

In this sentence, sound is the most neutral translation. Depending on context, it might also be understood as noise.

For example:

• Bir ses duydum. = I heard a sound / noise.
• Onun sesini duydum. = I heard his/her voice.

So context determines the best English word.

Yes, it can in the right context, because ses can also mean voice. But without context, many learners and translators would first understand it as a terrible sound or a horrible noise.

If the speaker clearly means a person’s voice, context usually makes that obvious.

Yes. Turkish word order is flexible, although the neutral order here is very natural:

• Korkunç bir ses duydum.

Other orders are possible for emphasis, such as:

• Bir ses duydum, korkunçtu. = I heard a sound; it was terrible.
• Duydum korkunç bir ses. = more marked/poetic/dramatic
• Ben korkunç bir ses duydum. = I heard a terrible sound

The version you have is a normal, straightforward way to say it.

A rough pronunciation guide is:

• korkunç ≈ kor-koonch

A few notes:

• o is a pure vowel, not like the English diphthong in many accents
• u sounds like oo in food
• ç is pronounced like ch in church

So korkunç sounds approximately like kor-koonch.

A rough pronunciation is:

• duydum ≈ doo-y-doom

More carefully:

• duy has a sound a bit like doo-y
• the u vowels are like oo in food

So the whole word is roughly doo-y-doom.

Yes. It is a fully complete sentence.

Even though there is no explicit subject like ben, the verb duydum already includes the subject I, so nothing is missing.

That is very normal in Turkish.