Сцену озвучили и добавили титры.

Because it is the direct object of озвучили.

• Dictionary form: сцена = scene
• In the sentence, the scene is the thing that was dubbed/voiced
• A feminine singular noun used as a direct object normally goes into the accusative case

So:

• сцена → nominative
• сцену → accusative

That is why the sentence begins with Сцену озвучили...

Russian often leaves out the subject when it is understood from the verb form.

Озвучили is past tense plural, so the hidden subject is basically они = they.

But here it usually does not mean a specific they that the speaker wants to identify. It is often an indefinite-personal use:

• Сцену озвучили = They dubbed the scene
• In natural English, this may sound more like The scene was dubbed

So Russian uses a plural verb with no explicit subject where English often uses a passive.

Озвучили is the past plural form of озвучить, a perfective verb.

Depending on context, озвучить can mean:

• to voice
• to dub
• to provide voice-over for
• to add sound to

In film/media contexts, it often means that voices or sound were added to something that did not have them before, or that it was dubbed into another language.

So in this sentence, озвучили suggests a completed action: the scene was successfully voiced/dubbed.

Because the sentence presents both actions as completed results.

• озвучили = they dubbed/voiced it
• добавили = they added them

Perfective verbs are very common when talking about finished production steps, especially in media or technical contexts.

Compare:

• Сцену озвучили и добавили титры.
  = The actions were completed.

• Сцену озвучивали и добавляли титры.
  = They were dubbing the scene and adding subtitles/captions, or they used to do that.

So the perfective here gives a clear done-and-finished meaning.

It actually is functioning as the direct object of добавили, but for this kind of noun the accusative plural looks the same as the nominative plural.

• Dictionary form: титры
• Accusative plural of an inanimate noun is usually the same as nominative plural

So:

• титры can be nominative plural
• титры can also be accusative plural

That is why the form does not visibly change.

Титры is a broad word for text shown on screen. Depending on context, it can mean things like:

• credits
• titles
• captions
• sometimes subtitles

If the meaning shown to the learner is subtitles, that is possible in context, but Russian also has the more specific word субтитры.

So:

• титры = broader, more general screen text
• субтитры = specifically subtitles

In real usage, context decides which translation is best.

Because Russian often leaves out information that is obvious from context.

In English, we might say:

• They added subtitles to the scene

In Russian, if it is already clear what the subtitles were added to, it is very natural to say simply:

• добавили титры

The target is understood from the previous part of the sentence:

• Сцену озвучили и добавили титры.

So the idea is basically:

• They dubbed the scene and added captions/subtitles [to it].

Russian often prefers this kind of compact phrasing.

Starting with Сцену puts focus on what was worked on.

Russian word order is flexible, and the beginning of the sentence often contains the topic or the thing being discussed. Here, the sentence is about the scene, and then it tells us what was done to it:

• Сцену озвучили и добавили титры.

A version with an explicit subject would also be possible:

• Они озвучили сцену и добавили титры.

But the original sounds more natural if the important point is the scene itself, not who did the actions.

So the fronting of Сцену is very normal and helps structure the information.