El actor con el papel protagonista proyecta la voz.

Because proyecta is the 3rd person singular form of the verb proyectar in the present tense.

• proyectar = to project
• él/ella proyecta = he/she projects

So in El actor con el papel protagonista proyecta la voz, the subject is El actor, so the verb must be proyecta.

Proyectar la voz is a common expression meaning to project your voice, especially so that people can hear you clearly in a theatre or large space.

It does not mean projecting something onto a screen here.
With voz, proyectar is about speaking strongly and clearly.

Spanish often uses the definite article (el, la, los, las) with body parts and things closely associated with a person when the owner is already obvious.

So:

• proyecta la voz = literally projects the voice
• but in natural English, we usually say projects his voice

Spanish prefers la voz here because it sounds more natural than su voz.

Here con means something like with or having.

So el actor con el papel protagonista means:

• the actor with the leading role
• or the actor who has the leading role

It describes which actor we are talking about.

In Spanish, adjectives usually come after the noun.

• papel = role
• protagonista = leading / main / protagonist

So:

• el papel protagonista = the leading role

Putting protagonista after the noun is the normal order here.

Both can relate to the idea of a main role, but they are not always exactly the same.

• papel protagonista = the role of the protagonist / leading role
• papel principal = main role / principal role

In many contexts they are very similar.
Protagonista focuses more on being the protagonist or central character, while principal means main more generally.

In this sentence, protagonista is functioning as an adjective modifying papel.

• el papel protagonista = the leading role

But protagonista can also be a noun:

• El protagonista de la obra = The protagonist of the play

So this is one of those words that can do both jobs depending on the sentence.

Because protagonista is a word with the same ending for both genders.

For example:

• el protagonista
• la protagonista

And as an adjective:

• el papel protagonista
• la actriz protagonista

The article or the noun around it usually shows the gender.

Because the sentence is talking about one actor.

• el actor = the actor
• los actores = the actors

Since the subject is singular, the verb is singular too:

• El actor proyecta
• Los actores proyectan

It means the actor literally, because el is the definite article.

However, in Spanish, el can sometimes be used in a more general sense too, depending on context. For example, it could refer to:

• a specific actor already mentioned, or
• a general type of actor in a certain role

Without more context, the actor is the best direct interpretation.

Yes, you might see something like that, but it changes the nuance a little.

• con el papel protagonista = the actor with the leading role / who has the leading role
• en el papel protagonista = the actor in the leading role

Both are possible depending on context, but con focuses on having the role, while en focuses more on appearing in that role.

Yes. This word order is natural:

• El actor = subject
• con el papel protagonista = phrase describing the actor
• proyecta = verb
• la voz = direct object

Spanish often places descriptive phrases like con el papel protagonista right after the noun they describe. So the sentence is structured very normally.